The dissection of 400 Australorbis olivaceiis from a brook situated in the grounds of the Instituto Oswaldo Cruz (Manguinhos) showed that 8.25 per cent of them were naturally infested with Schistosoma mansoni. The cercariae of 24 snails were inoculated into 24 guinea pigs, each of the latter receiving the cercariae from a single snail. A second series of inoculations with the cercariae of 6 surviving snails was made 10 days after the first series. All resulting infestations were unisexual (13 with males, 11 with females), a sex ratio of 1 : 1 being observed. In 6 cases infested with females, 15 per cent of the latter showed, either in the uterus or in the oötype, ovular bodies (figs. 1 to 15) destitute of embryonic structures, whose number ranged from 1 to 6. When laid in the liver, these eggs gave origin to a slight infiltrate of epithelioid cells, and did not reveal any embryonic organization. Eggs were not found in the sections of the intestines. The sexual organs of the females were apparently well deve¬loped in a large number of specimens. The males had apparently normal testes, whose number ranged from 4 to 9. Supernumerary testes were found in 1 per cent of the specimens exa¬mined. Males secondarily hermaphrodites were found in 2 cases (29 and 25 per cent, respectively).