diff --git a/src/user-groups/__tests__/userGroupsReducer-test.js b/src/user-groups/__tests__/userGroupsReducer-test.js
index 37aebe9bf85..b909838b50e 100644
--- a/src/user-groups/__tests__/userGroupsReducer-test.js
+++ b/src/user-groups/__tests__/userGroupsReducer-test.js
@@ -4,12 +4,14 @@ import deepFreeze from 'deep-freeze';
 
 import * as eg from '../../__tests__/lib/exampleData';
 import {
+  EVENT,
   EVENT_USER_GROUP_ADD,
   EVENT_USER_GROUP_REMOVE,
   EVENT_USER_GROUP_UPDATE,
   EVENT_USER_GROUP_ADD_MEMBERS,
   EVENT_USER_GROUP_REMOVE_MEMBERS,
 } from '../../actionConstants';
+import { EventTypes } from '../../api/eventTypes';
 import userGroupsReducer from '../userGroupsReducer';
 
 describe('userGroupsReducer', () => {
@@ -192,4 +194,35 @@ describe('userGroupsReducer', () => {
       ).toEqual([{ ...group1, members: [user1.user_id, user4.user_id] }, group2]);
     });
   });
+
+  describe('realm_user op: update', () => {
+    test('a user is deactivated', () => {
+      const user1 = eg.makeUser();
+      const user2 = eg.makeUser();
+      const user3 = eg.makeUser();
+
+      const group1 = eg.makeUserGroup({ members: [user1.user_id, user2.user_id, user3.user_id] });
+      const group2 = eg.makeUserGroup({ members: [user2.user_id, user1.user_id] });
+      const group3 = eg.makeUserGroup({ members: [user1.user_id] });
+
+      const person = { userToDeactivate: user1 };
+      const action = deepFreeze({
+        type: EVENT,
+        event: { id: 1, type: EventTypes.realm_user, op: 'update', person },
+      });
+
+      const prevState = deepFreeze([group1, group2, group3]);
+
+      const actualState = userGroupsReducer(prevState, action);
+
+      expect(actualState).toEqual([
+        { ...group1, members: [user2.user_id, user3.user_id] },
+        { ...group2, members: [user2.user_id] },
+
+        // A newly-empty group is not pruned; when a group is deactivated,
+        // we expect a user_group/remove event.
+        { ...group3, members: [] },
+      ]);
+    });
+  });
 });
diff --git a/src/user-groups/userGroupsReducer.js b/src/user-groups/userGroupsReducer.js
index 7ee86da4c7f..41b450de20f 100644
--- a/src/user-groups/userGroupsReducer.js
+++ b/src/user-groups/userGroupsReducer.js
@@ -2,6 +2,7 @@
 import type { UserGroupsState, PerAccountApplicableAction } from '../types';
 import {
   REGISTER_COMPLETE,
+  EVENT,
   EVENT_USER_GROUP_ADD,
   EVENT_USER_GROUP_REMOVE,
   EVENT_USER_GROUP_UPDATE,
@@ -9,6 +10,7 @@ import {
   EVENT_USER_GROUP_REMOVE_MEMBERS,
   RESET_ACCOUNT_DATA,
 } from '../actionConstants';
+import { EventTypes } from '../api/eventTypes';
 import { NULL_ARRAY } from '../nullObjects';
 
 const initialState: UserGroupsState = NULL_ARRAY;
@@ -69,6 +71,31 @@ export default (
     case EVENT_USER_GROUP_REMOVE_MEMBERS:
       return eventUserGroupRemoveMembers(state, action);
 
+    case EVENT: {
+      const { event } = action;
+      switch (event.type) {
+        case EventTypes.realm_user: {
+          switch (event.op) {
+            case 'update': {
+              const { person } = event;
+              if (person.userToDeactivate != null) {
+                const userId = person.userToDeactivate.user_id;
+                return state.map(g => ({ ...g, members: g.members.filter(m => m !== userId) }));
+              }
+
+              return state;
+            }
+
+            default:
+              return state;
+          }
+        }
+
+        default:
+          return state;
+      }
+    }
+
     default:
       return state;
   }