diff --git a/src/serialization/sb2.js b/src/serialization/sb2.js
index 5225614a4..c56479aae 100644
--- a/src/serialization/sb2.js
+++ b/src/serialization/sb2.js
@@ -52,6 +52,18 @@ const parseProcedureArgMap = function (procCode) {
     return argMap;
 };
 
+/**
+ * Generate a list of "argument IDs" for procdefs and caller mutations.
+ * IDs just end up being `input0`, `input1`, ... which is good enough.
+ * @param {string} procCode Scratch 2.0 procedure string.
+ * @return {Array.<string>} Array of argument id strings.
+ */
+const parseProcedureArgIds = function (procCode) {
+    return parseProcedureArgMap(procCode)
+        .map(arg => arg.inputName)
+        .filter(name => name); // Filter out unnamed inputs which are labels
+}
+
 /**
  * Flatten a block tree into a block list.
  * Children are temporarily stored on the `block.children` property.
@@ -501,6 +513,7 @@ const parseBlock = function (sb2block, getVariableId) {
                 tagName: 'mutation',
                 proccode: procData[0], // e.g., "abc %n %b %s"
                 argumentnames: JSON.stringify(procData[1]), // e.g. ['arg1', 'arg2']
+                argumentids: JSON.stringify(parseProcedureArgIds(procData[0])),
                 argumentdefaults: JSON.stringify(procData[2]), // e.g., [1, 'abc']
                 warp: procData[3], // Warp mode, e.g., true/false.
                 children: []
@@ -512,7 +525,8 @@ const parseBlock = function (sb2block, getVariableId) {
         activeBlock.mutation = {
             tagName: 'mutation',
             children: [],
-            proccode: sb2block[1]
+            proccode: sb2block[1],
+            argumentids: JSON.stringify(parseProcedureArgIds(sb2block[1]))
         };
     } else if (oldOpcode === 'getParam') {
         // Mutation for procedure parameter.