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Functioning as fully decentralised distributed systems, without the need of predefined infrastructures, mobile ad hoc networks provide interesting solutions when setting up dynamic and flexible applications. However, these systems also bring up some problems. In such open environments, it is difficult to discover among the nodes, which are malicious and which are not, in order to be able to choose good partners for cooperation. One solution for this to be possible, is for the entities to be able to evaluate the trust they have in each other and, based on this trust, determine which entities they can cooperate with. In this paper, we present a trust model adapted to ad hoc networks and, more generally, to distributed systems. This model is based on Bayesian networks, a probabilistic tool which provides a flexible means of dealing with probabilistic problems involving causality. The model evaluates the trust in a server according, both, to direct experiences with the server and recommendations concerning its service. We show, through a simulation, that the proposed model can determine the best server out of a set of eligible servers offering a given service. Such a trust model, when applied to ad hoc networks, tends to increase the QoS of the various services used by a host. This, when applied to security related services thus increases the overall security of the hosts.