Trust is a concept from the Social Sciences and can be defined as how much a node is willing to take the risk of trusting another one. The correct evaluation of the trust is crucial for several security mechanisms for Mobile Ad Hoc Networks (MANETs). However, the implementation of an effective trust evaluation scheme is very difficult in such networks, due to their dynamic characteristics. This work presents a trust evaluation scheme for MANETs based on a self-organized virtual trust network. To estimate the trustworthiness of other nodes, nodes form trust chains based on behavior evidences maintained within the trust network. Nodes periodically exchange their trust networks with the neighbors, providing an efficient method to disseminate trust information across the network. The scheme is fully distributed and self-organized, not requiring any trusted third party. Simulation results show that the scheme is very efficient on gathering evidences to build the trust networks. It also shows that the scheme has a very small communication and memory overhead. Besides, it is the first trust evaluation scheme evaluated under bad mouthing and newcomers attacks and it maintains its effectiveness in such scenarios.