Abstract. A major problem with the on-demand routing protocols for MANETs is the high cost flooding associated with route discovery process. Amongst many different optimizations for the pure flooding, the simple epidemic algorithm that emulates the spread of an infection in a crowded population, is one of the promising approaches. Based on the observation that phase transition phenomenon occurs under the relatively realistic ad hoc conditions with modest node mobility, our study show that adapting the packet retransmission probability to the local topology information can greatly enhance the overall performance of on-demand routing protocols for MANETs. The simulation results also exhibit that the adaptive gossip-based flooding offers a reasonable scalability in a relatively large-scale network.