Intrusion detection, as a complementary mechanism to intrusion prevention, is necessary to secure wireless Mobile Ad hoc Networks (MANETs). In this paper we propose a practical agent-based distributed intrusion detection methodology for MANETs. A two-step intrusion detection procedure has been developed to effectively detect anomalies and identify attack types using distributed intrusion detection agents. The approach is efficient in dealing with large amount of system audit information with the growing network size. In addition, the distributed agent based implementation provides inherent flexibility and scalability. The performance of the approach has been evaluated via extensive simulations.