A sero-immunological survey by immunofluorescnece has been conducted on 94 rats (R. rattus and R. norvegicus) captured in Guadeloupe, in there different biotopes where the presence of S. mansoni carriers had been reported. These biotopes are those of the "Grand Etang" (Great Pond), the fresh-water mangrove and its back-mangrove area, and the Beaugendre valley. The post-mortem performed on the rats revealed that 92 % of those carrying. S. mansoni eggs of full-grown worms, had a rate of antibodies superior to 40. The rates of antibodies obtained, as well as the percentages of S. mansoni-parasited rats vary according to the biotopes. Two hypotheses can account for these variations: a smaller or greater adaptation of the S. mansoni stock to the rat; a change in the comportment of rats which would thus be more or exposed to reinfestations.