The visceral mass of Biomphalaria glabrata uninfected or infected with Schistosoma mansoni was serially sectioned. The amount of hepatopancreas tissue and of parasite tissue was quantified. In pool-infected snails the volume of the whole visceral mass increased very significantly until week 6 and then decreased. Due to the growing parasites the volume of the visceral mass in infected snails was at most times higher when compared with uninfected snails of the same dimensions. Two weeks p.i. there was a permanent and significant increase of parasite tissue until week 6. During this time the amount of hepatopancreas tissue still increased. From this time onwards till week 12 the proportion of parasite tissue remained rather constant, indicating a kind of regulation, whereas the hepatopancreas tissue decreased to about one-third of the volume found in uninfected snails of the same shell diameter. Compared with infections by a single miracidium there was a significant increase in parasite tissue after infection with two miracidia. Infections with more miracidia (5, 10, and 20) gave no significant further increase. This also demonstrates a kind of regulation. Mortality rate, growth rate and egg production were studied during an infection period of 12 weeks.