The development of S. mansoni in 2 natural hosts, R. rattus and R. norvegicus, and in 2 laboratory hosts, the white mouse and the white rat, was studied. The survival rate, i.e., the percentage of cercariae recovered as adult worms 4 wk after exposure, was 31% in R. rattus, 27% in the white mouse, 14% in the laboratory rat and 12% in R. norvegicus. After 20 wk, 62% of the schistosomes which were present 4 wk after exposure were still alive in the mouse, 48% in R. rattus, 8% in R. norvegicus and 5% in the laboratory rat. The average size of males 8 wk after exposure was 7 mm in the mouse, 5.1 mm in R. rattus, 2.9 mm in the white rat and 2.6 mm in R. norvegicus. S. mansoni laid eggs in the 4 rodents; only mice and R. rattus eliminated fertile eggs containing infectious miracidia of S. mansoni; in R. norvegicus, as in the white rat, the eggs of S. mansoni were not fertile and were never eliminated by the host. Susceptibility may not always be associated with permissivity; there was a great difference between the 2 natural hosts: R. norvegicus accepts S. mansoni but did not allow its transmission; R. rattus, which was more favorable to growth and reproduction of S. mansoni, allowed the transit and exit of fertile eggs. Apparently, R. rattus was able to play a part in the dynamics of the Guadeloupean schistosomiasis focus while R. norvegicus is unable to contribute to the circulation of S. mansoni and is a decoy host for the parasite.