In mobile ad hoc networks (MANETs), a source node must rely on other nodes to forward its packets on multi-hop routes to the destination. Secure and reliable handling of packets by the intermediate nodes is difficult to ensure in an ad hoc network. This is particularly challenging when malicious nodes collude with one another to disrupt the network operation. So we propose a trust establishment scheme for MANETs which aims to improve the reliability of packet forwarding over multi-hop routes in the presence of potentially malicious nodes using mobile agent (MA). We evaluate two key metric for trust establishment namely trust and confidence which are used by corresponding MA for route discovery and topology maintenance. We have extended the solution to secure the network against colluding malicious nodes. Proposed scheme is evaluated using PMADE (platform for mobile agent distribution and execution) along with Glomosim. Simulation results shows that proposed scheme is quite effective for trust establishment in ad hoc networks.