During the flooding of primary lowland rain forest by the filling of ahydroelectric reservoir in French Guiana, live-trapping was conducted on 175subsequently formed islands. Different factors affecting the mammal trapefficiency were analysed. The main results show the real complexity of thesampling problem and the influence of the habitat disturbance due to theflooding. The location and/or the type of traps influenced captures ofDasypus novemcinctus, Proechimys spp., Metachirus nudicaudatus and Philanderopossum 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 speciesrichness. The number of captures increased with the number of trapnights duringthe first water inflow and the level stretch. The best trapping success was observedduring the wet season, when the strong habitat modification and the forest fragmentation became more significant. It resulted from the reduction of availableland area and trophic resources. One year after the beginning of the water inflow, the decrease of the number of captures with the increasingnumber of trapnights reflected a real decrease of the number of mammals onthe islands. The species richness increased with the number of trapnights inall periods, and was also the highest during the wet season.