diff --git a/src/editor/editor.ts b/src/editor/editor.ts
index 7d0c21fc..d634a986 100644
--- a/src/editor/editor.ts
+++ b/src/editor/editor.ts
@@ -31,9 +31,9 @@ import * as eventHandler from '../blocks/mrc_event_handler';
 import * as classMethodDef from '../blocks/mrc_class_method_def';
 import * as mechanismComponentHolder from '../blocks/mrc_mechanism_component_holder';
 //import { testAllBlocksInToolbox } from '../toolbox/toolbox_tests';
-import { getToolboxJSON } from '../toolbox/toolbox';
+import { applyExpandedCategories, getToolboxJSON } from '../toolbox/toolbox';
 
-const EMPTY_TOOLBOX: Blockly.utils.toolbox.ToolboxDefinition = {
+const EMPTY_TOOLBOX: Blockly.utils.toolbox.ToolboxInfo = {
   kind: 'categoryToolbox',
   contents: [],
 };
@@ -58,7 +58,7 @@ export class Editor {
   private mechanismClassNameToModuleContent: {[mechanismClassName: string]: storageModuleContent.ModuleContent} = {};
   private bindedOnChange: any = null;
   private shownPythonToolboxCategories: Set<string> | null = null;
-  private toolbox: Blockly.utils.toolbox.ToolboxDefinition = EMPTY_TOOLBOX;
+  private toolbox: Blockly.utils.toolbox.ToolboxInfo = EMPTY_TOOLBOX;
 
   constructor(
       blocklyWorkspace: Blockly.WorkspaceSvg,
@@ -199,6 +199,10 @@ export class Editor {
       return;
     }
     const toolbox = getToolboxJSON(this.shownPythonToolboxCategories, this);
+    const previousToolbox = this.blocklyWorkspace.getToolbox();
+    if (previousToolbox) {
+      applyExpandedCategories(previousToolbox, toolbox);
+    }
     if (toolbox != this.toolbox) {
       this.toolbox = toolbox;
       this.blocklyWorkspace.updateToolbox(toolbox);
diff --git a/src/toolbox/toolbox.ts b/src/toolbox/toolbox.ts
index 998d6bfa..a3a539af 100644
--- a/src/toolbox/toolbox.ts
+++ b/src/toolbox/toolbox.ts
@@ -8,9 +8,9 @@ import { getCategory as getEventCategory } from './event_category';
 
 export function getToolboxJSON(
     shownPythonToolboxCategories: Set<string> | null,
-    editor: Editor): Blockly.utils.toolbox.ToolboxDefinition {
+    editor: Editor): Blockly.utils.toolbox.ToolboxInfo {
 
-  const toolbox: Blockly.utils.toolbox.ToolboxDefinition = {
+  const toolbox: Blockly.utils.toolbox.ToolboxInfo = {
     kind: 'categoryToolbox',
     contents: []
   };
@@ -34,3 +34,70 @@ export function getToolboxJSON(
   // This trouble is prevented by stringifying and parsing.
   return JSON.parse(JSON.stringify(toolbox));
 }
+
+/**
+ * Collects the expanded/collapsed state of categories in the previous toolbox and applies them
+ * to the new toolbox.
+ */
+export function applyExpandedCategories(
+    previousToolbox: Blockly.IToolbox,
+    newToolbox: Blockly.utils.toolbox.ToolboxInfo) {
+  const expanded: {[categoryPath: string]: boolean} = {};
+  recursivelyCollectExpandedCategories((previousToolbox as any).getToolboxItems(), expanded, '', null);
+  recursivelyApplyExpandedCategories(newToolbox.contents, expanded, '');
+}
+
+/**
+ * Collects the expanded/collapsed state for all collapsible items.
+ */
+function recursivelyCollectExpandedCategories(
+    items: Blockly.IToolboxItem[],
+    expanded: {[categoryPath: string]: boolean},
+    parentPath: string,
+    parent: Blockly.IToolboxItem | null) {
+  items
+      .filter(item => item.getParent() == parent)
+      .filter(item => item.isCollapsible())
+      .forEach(item => {
+        const collapsibleItem = item as Blockly.ICollapsibleToolboxItem;
+        const path = makePath(parentPath, collapsibleItem.getName());
+        expanded[path] = collapsibleItem.isExpanded();
+        recursivelyCollectExpandedCategories(
+            collapsibleItem.getChildToolboxItems(), expanded, path, item);
+      });
+}
+
+/**
+ * Applies previously collected expanded/collapsed state to the given toolbox categories.
+ */
+function recursivelyApplyExpandedCategories(
+    contents: toolboxItems.ContentsType[],
+    expanded: {[categoryPath: string]: boolean},
+    parentPath: string) {
+  contents
+      .filter(item => item.kind === 'category')
+      .forEach(item => {
+        const category = item as toolboxItems.Category;
+        const path = makePath(parentPath, category.name);
+        if (path in expanded) {
+          if (expanded[path]) {
+            category.expanded = true;
+          } else {
+            delete category.expanded;
+          }
+        }
+        if (category.contents) {
+          recursivelyApplyExpandedCategories(category.contents, expanded, path);
+        }
+      });
+}
+
+/**
+ * Returns a string that combines the given parentPath with the given child.
+ */
+function makePath(parentPath: string, child: string) {
+  if (parentPath) {
+    return parentPath + '\u2705' + child;
+  }
+  return child;
+}
diff --git a/src/toolbox/toolbox_tests.ts b/src/toolbox/toolbox_tests.ts
index 586676e7..9b0d7d1d 100644
--- a/src/toolbox/toolbox_tests.ts
+++ b/src/toolbox/toolbox_tests.ts
@@ -25,8 +25,8 @@ import * as toolboxItems from './items';
 
 // Tests
 
-export function testAllBlocksInToolbox(toolbox : Blockly.utils.toolbox.ToolboxDefinition) {
-  const contents  = (toolbox as toolboxItems.Category).contents;
+export function testAllBlocksInToolbox(toolbox : Blockly.utils.toolbox.ToolboxInfo) {
+  const contents  = toolbox.contents;
   alert('Press OK to run tests on all blocks from the toolbox.');
   const toolboxTestData = new ToolboxTestData(contents, () => {
     alert('Completed tests on all blocks in the toolbox. See console for any errors.');