This paper is concerned with the concurrency control for collaborative virtual environments. In particular, we describe how concurrent actions are coordinated in a multi-user, large-scale 3-D layout system CIAO. In contrast to many existing systems that sacrifice responsiveness in order to maintain consistency, CIAO achieves optimal response and notification time without compromising consistency. The optimal responsiveness is achieved by a new multicast-based, optimistic concurrency control mechanism. Even operations on a group of related objects do not entail any latency for concurrency control. We also present the multi-user interfaces of CIAO that provide some sense of isolation as well as rich awareness.