diff --git a/question/format.php b/question/format.php
index 5e82530a454..0177abf03e4 100644
--- a/question/format.php
+++ b/question/format.php
@@ -575,14 +575,22 @@ class qformat_default {
                 $parent = $category->id;
             } else if ($category = $DB->get_record('question_categories',
                     array('name' => $catname, 'contextid' => $context->id, 'parent' => $parent))) {
-                // Do nothing unless the child category appears before the parent category
-                // in the imported xml file. Because the parent was created without info being available
-                // at that time, this allows the info to be added from the xml data.
-                if ($key == (count($catnames) - 1) && $lastcategoryinfo && isset($lastcategoryinfo->info) &&
-                        $lastcategoryinfo->info !== '' && $category->info === '') {
-                    $category->info = $lastcategoryinfo->info;
-                    if (isset($lastcategoryinfo->infoformat) && $lastcategoryinfo->infoformat !== '') {
-                        $category->infoformat = $lastcategoryinfo->infoformat;
+                // If this category is now the last one in the path we are processing ...
+                if ($key == (count($catnames) - 1) && $lastcategoryinfo) {
+                    // Do nothing unless the child category appears before the parent category
+                    // in the imported xml file. Because the parent was created without info being available
+                    // at that time, this allows the info to be added from the xml data.
+                    if (isset($lastcategoryinfo->info) && $lastcategoryinfo->info !== ''
+                            && $category->info === '') {
+                        $category->info = $lastcategoryinfo->info;
+                        if (isset($lastcategoryinfo->infoformat) && $lastcategoryinfo->infoformat !== '') {
+                            $category->infoformat = $lastcategoryinfo->infoformat;
+                        }
+                    }
+                    // Same for idnumber.
+                    if (isset($lastcategoryinfo->idnumber) && $lastcategoryinfo->idnumber !== ''
+                            && $category->idnumber === '') {
+                        $category->idnumber = $lastcategoryinfo->idnumber;
                     }
                     $DB->update_record('question_categories', $category);
                 }
@@ -603,11 +611,16 @@ class qformat_default {
                 $category->name = $catname;
                 $category->info = '';
                 // Only add info (category description) for the final category in the catpath.
-                if ($key == (count($catnames) - 1) && $lastcategoryinfo && isset($lastcategoryinfo->info) &&
-                        $lastcategoryinfo->info !== '') {
-                    $category->info = $lastcategoryinfo->info;
-                    if (isset($lastcategoryinfo->infoformat) && $lastcategoryinfo->infoformat !== '') {
-                        $category->infoformat = $lastcategoryinfo->infoformat;
+                if ($key == (count($catnames) - 1) && $lastcategoryinfo) {
+                    if (isset($lastcategoryinfo->info) && $lastcategoryinfo->info !== '') {
+                        $category->info = $lastcategoryinfo->info;
+                        if (isset($lastcategoryinfo->infoformat) && $lastcategoryinfo->infoformat !== '') {
+                            $category->infoformat = $lastcategoryinfo->infoformat;
+                        }
+                    }
+                    // Same for idnumber.
+                    if (isset($lastcategoryinfo->idnumber) && $lastcategoryinfo->idnumber !== '') {
+                        $category->idnumber = $lastcategoryinfo->idnumber;
                     }
                 }
                 $category->parent = $parent;
@@ -924,7 +937,7 @@ class qformat_default {
                         if (!count($DB->get_records('question', array('category' => $trackcategoryparent)))) {
                             $categoryname = $this->get_category_path($trackcategoryparent, $this->contexttofile);
                             $categoryinfo = $DB->get_record('question_categories', array('id' => $trackcategoryparent),
-                                'name, info, infoformat', MUST_EXIST);
+                                'name, info, infoformat, idnumber', MUST_EXIST);
                             if ($categoryinfo->name != 'top') {
                                 // Create 'dummy' question for parent category.
                                 $dummyquestion = $this->create_dummy_question_representing_category($categoryname, $categoryinfo);
@@ -937,7 +950,7 @@ class qformat_default {
                 if ($addnewcat && !in_array($trackcategory, $writtencategories)) {
                     $categoryname = $this->get_category_path($trackcategory, $this->contexttofile);
                     $categoryinfo = $DB->get_record('question_categories', array('id' => $trackcategory),
-                            'info, infoformat', MUST_EXIST);
+                            'info, infoformat, idnumber', MUST_EXIST);
                     // Create 'dummy' question for category.
                     $dummyquestion = $this->create_dummy_question_representing_category($categoryname, $categoryinfo);
                     $expout .= $this->writequestion($dummyquestion) . "\n";
@@ -986,6 +999,7 @@ class qformat_default {
         $dummyquestion->contextid = 0;
         $dummyquestion->info = $categoryinfo->info;
         $dummyquestion->infoformat = $categoryinfo->infoformat;
+        $dummyquestion->idnumber = $categoryinfo->idnumber;
         $dummyquestion->name = 'Switch category to ' . $categoryname;
         return $dummyquestion;
     }
diff --git a/question/format/xml/format.php b/question/format/xml/format.php
index 98fe6d2c725..d2323d00afd 100644
--- a/question/format/xml/format.php
+++ b/question/format/xml/format.php
@@ -935,6 +935,7 @@ class qformat_xml extends qformat_default {
             // The import should have the format in human readable form, so translate to machine readable format.
             $qo->infoformat = $this->trans_format($question['#']['info'][0]['@']['format']);
         }
+        $qo->idnumber = $this->getpath($question, array('#', 'idnumber', 0, '#'), null);
         return $qo;
     }
 
@@ -1205,6 +1206,7 @@ class qformat_xml extends qformat_default {
             $expout .= "    <info {$infoformat}>\n";
             $expout .= "      {$categoryinfo}";
             $expout .= "    </info>\n";
+            $expout .= "    <idnumber>{$question->idnumber}</idnumber>\n";
             $expout .= "  </question>\n";
             return $expout;
         }
diff --git a/question/format/xml/tests/fixtures/category_with_description.xml b/question/format/xml/tests/fixtures/category_with_description.xml
index 5c8ff8331af..674457bab42 100644
--- a/question/format/xml/tests/fixtures/category_with_description.xml
+++ b/question/format/xml/tests/fixtures/category_with_description.xml
@@ -8,6 +8,7 @@
     <info format="moodle_auto_format">
         <text>This is Alpha category for test</text>
     </info>
+    <idnumber>alpha-idnumber</idnumber>
   </question>
 
 <!-- question: 91  -->
diff --git a/question/format/xml/tests/fixtures/export_category.xml b/question/format/xml/tests/fixtures/export_category.xml
index 42c9f37e57f..63087cf7264 100644
--- a/question/format/xml/tests/fixtures/export_category.xml
+++ b/question/format/xml/tests/fixtures/export_category.xml
@@ -8,6 +8,7 @@
     <info format="moodle_auto_format">
       <text>This is Alpha category for test</text>
     </info>
+    <idnumber>alpha-idnumber</idnumber>
   </question>
 
 <!-- question: 91  -->
diff --git a/question/format/xml/tests/fixtures/nested_categories.xml b/question/format/xml/tests/fixtures/nested_categories.xml
index 4bf5c2cfca6..ac6db64d315 100644
--- a/question/format/xml/tests/fixtures/nested_categories.xml
+++ b/question/format/xml/tests/fixtures/nested_categories.xml
@@ -8,6 +8,7 @@
     <info format="plain_text">
       <text>This is Delta category for test</text>
     </info>
+    <idnumber></idnumber>
   </question>
 
 <!-- question: 0  -->
@@ -18,6 +19,7 @@
     <info format="markdown">
       <text>This is Epsilon category for test</text>
     </info>
+    <idnumber></idnumber>
   </question>
 
 <!-- question: 0  -->
@@ -28,6 +30,7 @@
     <info format="moodle_auto_format">
       <text>This is Zeta category for test</text>
     </info>
+    <idnumber></idnumber>
   </question>
 
 <!-- question: 93  -->
diff --git a/question/format/xml/tests/fixtures/nested_categories_with_questions.xml b/question/format/xml/tests/fixtures/nested_categories_with_questions.xml
index db729e99dc9..8350c16c4f8 100644
--- a/question/format/xml/tests/fixtures/nested_categories_with_questions.xml
+++ b/question/format/xml/tests/fixtures/nested_categories_with_questions.xml
@@ -8,6 +8,7 @@
     <info format="plain_text">
       <text>This is Iota category for test</text>
     </info>
+    <idnumber></idnumber>
   </question>
 
 <!-- question: 96  -->
@@ -47,6 +48,7 @@
     <info format="markdown">
       <text>This is Kappa category for test</text>
     </info>
+    <idnumber></idnumber>
   </question>
 
 <!-- question: 106  -->
@@ -114,6 +116,7 @@
     <info format="moodle_auto_format">
       <text>This is Lambda category for test</text>
     </info>
+    <idnumber></idnumber>
   </question>
 
 <!-- question: 98  -->
@@ -153,6 +156,7 @@
     <info format="moodle_auto_format">
       <text>This is Mu category for test</text>
     </info>
+    <idnumber></idnumber>
   </question>
 
 <!-- question: 99  -->
diff --git a/question/format/xml/tests/qformat_xml_import_export_test.php b/question/format/xml/tests/qformat_xml_import_export_test.php
index a0af0afd865..4926bea1040 100644
--- a/question/format/xml/tests/qformat_xml_import_export_test.php
+++ b/question/format/xml/tests/qformat_xml_import_export_test.php
@@ -38,8 +38,8 @@ class qformat_xml_import_export_test extends advanced_testcase {
     /**
      * Create object qformat_xml for test.
      * @param string $filename with name for testing file.
-     * @param object $course
-     * @return object qformat_xml.
+     * @param stdClass $course
+     * @return qformat_xml XML question format object.
      */
     public function create_qformat($filename, $course) {
         global $DB;
@@ -99,11 +99,12 @@ class qformat_xml_import_export_test extends advanced_testcase {
      * @param string $info imported category info field (description of category).
      * @param int $infoformat imported category info field format.
      */
-    public function assert_category_imported($name, $info, $infoformat) {
+    public function assert_category_imported($name, $info, $infoformat, $idnumber = null) {
         global $DB;
         $category = $DB->get_record('question_categories', ['name' => $name], '*', MUST_EXIST);
         $this->assertEquals($info, $category->info);
         $this->assertEquals($infoformat, $category->infoformat);
+        $this->assertSame($idnumber, $category->idnumber);
     }
 
     /**
@@ -146,7 +147,8 @@ class qformat_xml_import_export_test extends advanced_testcase {
         $qformat = $this->create_qformat('category_with_description.xml', $course);
         $imported = $qformat->importprocess();
         $this->assertTrue($imported);
-        $this->assert_category_imported('Alpha', 'This is Alpha category for test', FORMAT_MOODLE);
+        $this->assert_category_imported('Alpha',
+                'This is Alpha category for test', FORMAT_MOODLE, 'alpha-idnumber');
         $this->assert_category_has_parent('Alpha', 'top');
     }
 
@@ -246,6 +248,7 @@ class qformat_xml_import_export_test extends advanced_testcase {
                 'contextid' => '2',
                 'info' => 'This is Alpha category for test',
                 'infoformat' => '0',
+                'idnumber' => 'alpha-idnumber',
                 'stamp' => make_unique_id_code(),
                 'parent' => '0',
                 'sortorder' => '999']);
diff --git a/question/format/xml/tests/xmlformat_test.php b/question/format/xml/tests/xmlformat_test.php
index c85fff02b26..c8cd52c41f8 100644
--- a/question/format/xml/tests/xmlformat_test.php
+++ b/question/format/xml/tests/xmlformat_test.php
@@ -1721,6 +1721,7 @@ END;
         $categoryinfo = new stdClass();
         $categoryinfo->info = 'info1';
         $categoryinfo->infoformat = 'infoformat1';
+        $categoryinfo->idnumber = null;
         $dummyquestion = $testobject->mock_create_dummy_question_representing_category($categoryname, $categoryinfo);
 
         $this->assertEquals('category', $dummyquestion->qtype);