Multicast communications are crucial to support collaborative information processing and dissemination in ad hoc sensor networks (ASNs). The unique characteristics, including the severe resource constraints of sensor nodes and the lack of infrastructure, make it highly challenging to secure the multicast communications in these networks. To this end, we propose a novel key pre-distribution based scheme, referred to as GKM, for secure multicasting in ASNs. Prior to network deployment, each node is assigned with a set of keys. After nodes are deployed and a multicast group is formed, a group tree is distributively constructed among the group members, by utilizing group dynamics such as hop distance between group members. The group keys are then established and updated through the group tree. Unlike existing key pre-distribution schemes, GKM does not rely on non-group intermediate nodes to manage group keys dynamically. As a result, GKM achieves higher levels of security than existing key pre-distribution schemes.