Wireless ad hoc networks have generated much interest, both in research literature and the telecommunication industry. The attractiveness of these networks lies in the fact that unlike other wireless networks, ad hoc networks are self-organized: the hosts constituting the networks can communicate with each other without reliance on centralized or specialized entities such as base stations. As these networks find more application, the need for adequate security mechanism is increasingly becoming important. Trust establishment and management are essential for any security framework of these networks. In this paper, we present a trust establishment scheme for ad hoc networks based on distributed trust model. A trust initiator is introduced only in the system-bootstrapping phase to initiate the protocol. A fully self-organized trust establishment approach is then adopted to handle the dynamic topology of the network and the membership changes of the nodes, while ensuring trust establishment among the nodes with shorter trust chains and very high probability. The simulation results show that our scheme is highly robust and scalable in the dynamic environment of ad hoc networks.