A mobile computing environment allows hosts to roam while retaining access to the Internet. Multicasting is one of the most important facilities for constructing reliable distributed systems and cooperative applications. Host mobility, however, challenges multicasting in this environment: the established multicast delivery paths may frequently restructure along with host migrations, incurring expensive overheads. This paper presents a framework for network-layer multicasting while keeping a low overhead in adapting multicast routes to mobile host locations. This is achieved by partitioning the mobile environment into non-overlapping regions, so that changes in the multicast routes due to host intra-region movements are hidden from other regions. An analytical model was developed for performance evaluation. It shows that, compared with the best known proposal, our scheme reduces the average multicast latency by more than 66%, while causing less than 7% overhead.