Abstract—Designing protocols for solving the consensus problem faces new challenges in mobile computing environments. Among others, how we can achieve message efficiency for saving resource consumption has been the focus of research. In this paper, we present the HC protocol, a message efficient consensus protocol for MANETs. We consider the widely used system model where the hosts fail by crashes and the system is equipped with Chandra-Toueg's unreliable failure detectors. Unlike existing consensus protocols, the HC protocol uses a two-layer hierarchy based on clusters to achieve message efficiency. The messages from and to the hosts in the same cluster are merged so as to reduce the message cost. However, adding such a hierarchy is not trivial. Due to host movements and failures, the hierarchy changes from time to time and this may cause message loss. In designing HC, we also propose methods to handle such message losses. Extensive simulations have been carried out to evaluate and compare the performance of the HC protocol and similar protocols in a MANET environment. Simulation results show that, in most cases, our protocol can significantly reduce both the message cost and time cost. With increases in the system scale or the percentage of faulty hosts, the advantage of our protocol becomes more obvious.