During the flooding of primary lowland rain forest by the filling of a hydroelectric reservoir in French Guiana, live-trapping was conducted on 175 subsequently formed islands. Different factors affecting the mammal trap efficiency were analysed. The main results show the real complexity of the sampling problem and the influence of the habitat disturbance due to the flooding. The location and/or the type of traps influenced captures of Dasypus novemcinctus, Proechimys spp., Metachirus nudicaudatus and Philander opossum in relation to their foraging and/or locomotion behaviour, and to to their size. The conformation of the islands (height and size) had no influence on the number of captures, nor on the species richness. The number of captures increased with the number of trapnights during the first water inflow and the level stretch. The best trapping success was observed during the wet season, when the strong habitat modification and the forest fragmentation became more significant. It resulted from the reduction of available land area and trophic resources. One year after the beginning of the water inflow, the decrease of the number of captures with the increasing number of trapnights reflected a real decrease of the number of mammals on the islands. The species richness increased with the number of trapnights in all periods, and was also the highest during the wet season.