A comparative study of the development of Schistosoma mansoni during the intra-molluscan phase was made by means of histological sections of Biomphalaria tenagophila, B. straminea and B. glabrata from Brazil. Two hundred snails of each species were individually exposed to 50 miracidia of the S. mansoni, AL line. No larvae were observed in the snails fixed 72 h after exposure. In specimens shedding cercariae, 31 days after exposure tissue reactions encapsulating the larvae were seen in B. tenagophila and B. straminea, in the head-foot, mantle collar and renal ducts. No tissue reactions occurred in the digestive glands of these two species. In B. glabrata the presence of numerous sporocysts and cercariae without tissue reactions was observed in the digestive gland, and other organs. The levels of infection of the snails and the average numbers of cercariae shed per day were 32.6% and 79 +/- 90 respectively for B. tenagophila, 11.3% and 112 +/- 100 for B. straminea and 75.3% and 432 +/- 436 for B. glabrata. The lower levels of infection and average numbers of cercariae shed by B. tenagophila and B. straminea are thus related to their more potent internal defense systems.