Wireless Mobile Ad hoc NETwork (WMANET) is a group of independent wireless (Mobile/ Semi Mobile) nodes communicating on a peer-to-peer basis with no pre- established infrastructure. The unique characteristics of WMANET make such networks highly vulnerable to security attacks when compared with wired networks or infrastructure-based wire less networks. This paper proposes a novel security algorithm for managing digital certificates when all WMANET nodes are Full y Managed (FM) by other heterogeneous infrastructure-based wireless networks such as WLANs and cellular systems. The new security algorithm, which is called FM-WMANET, examines the integration of heterogeneous wireless networks to enhance the performance of WMANETs from the security perspectives. FM-WMANET presents a novel integrated key mana gement system that is based on the hierarchal trust models used by the Public Key Infrastructures (PKIs) of extant heterogeneous wireless networks. FM-WMANET is still fully distributed and provides a high level of securi ty, availability, flexibility and efficiency key management services for WMANETs. Graph theory a nd network simulator NS-2 are used to represent the security system proposed by FM-WMANET, study its performance and demonstrate its effectiveness.