In mobile ad hoc networks (MANETs), many applications require group-oriented computing among a large number of nodes in an adversarial environment. To deploy these large-scale cooperative applications, secure multicast service must be provided to efficiently and safely exchange data among nodes. The existing literature has extensively studied security protection for a single multicast group, in which all nodes are assumed to have the same security level. However, such an assumption may not be valid in practice because, for many applications, different users can play different roles and thus naturally be classified into multiple security levels. In this paper, we propose a pyramidal security model to safeguard the multisecurity-level information sharing in one cooperation domain. As a prominent feature, a pyramidal security model contains a set of hierarchical security groups and multicast groups. To find an efficient key management solution that covers all the involved multicast groups, we develop the following three schemes for the proposed security model: (1) separated star key graph; (2) separated tree key graph, and (3) integrated tree key graph. Performance comparison demonstrates that the scheme of integrated tree key graph has advantages over its counterparts.