diff --git a/blocks/procedures.js b/blocks/procedures.js
index 82b383b51..19a5f6e68 100644
--- a/blocks/procedures.js
+++ b/blocks/procedures.js
@@ -400,14 +400,20 @@ Blockly.Blocks['procedures_mutatorarg'] = {
    * @this Blockly.Block
    */
   init: function() {
+    var field = new Blockly.FieldTextInput('x', this.validator_);
     this.appendDummyInput()
         .appendField(Blockly.Msg.PROCEDURES_MUTATORARG_TITLE)
-        .appendField(new Blockly.FieldTextInput('x', this.validator_), 'NAME');
+        .appendField(field, 'NAME');
     this.setPreviousStatement(true);
     this.setNextStatement(true);
     this.setColour(Blockly.Blocks.procedures.HUE);
     this.setTooltip(Blockly.Msg.PROCEDURES_MUTATORARG_TOOLTIP);
     this.contextMenu = false;
+
+    // Create the default variable when we drag the block in from the flyout.
+    // Have to do this after installing the field on the block.
+    field.onFinishEditing_ = this.createNewVar_;
+    field.onFinishEditing_('x');
   },
   /**
    * Obtain a valid name for the procedure.
@@ -421,6 +427,20 @@ Blockly.Blocks['procedures_mutatorarg'] = {
   validator_: function(newVar) {
     newVar = newVar.replace(/[\s\xa0]+/g, ' ').replace(/^ | $/g, '');
     return newVar || null;
+  },
+  /**
+   * Called when focusing away from the text field.
+   * Creates a new variable with this name.
+   * @param {string} newText The new variable name.
+   * @private
+   * @this Blockly.FieldTextInput
+   */
+  createNewVar_: function(newText) {
+    var source = this.sourceBlock_;
+    if (source && source.workspace && source.workspace.options
+        && source.workspace.options.parentWorkspace) {
+      source.workspace.options.parentWorkspace.createVariable(newText);
+    }
   }
 };
 
diff --git a/core/field_textinput.js b/core/field_textinput.js
index ba9375e4c..859244bba 100644
--- a/core/field_textinput.js
+++ b/core/field_textinput.js
@@ -269,6 +269,9 @@ Blockly.FieldTextInput.prototype.widgetDispose_ = function() {
       } else {
         // Validation function has changed the text.
         text = text1;
+        if (thisField.onFinishEditing_) {
+          thisField.onFinishEditing_(text);
+        }
       }
     }
     thisField.setValue(text);