The growth and demography of the freshwater snail Biomphalaria pfeifferi, an intermediate host of the parasitic worm Schistosoma mansoni in Eastern Zaire, were investigated in the laboratory under two conditions of crowding. Both individual and population growth were depressed by crowding. Crowding reduced growth rate and all the variables and parameters related to reproduction (fecundity, instantaneous birth rate, net reproduction rate), but did not affect maximum size, survivorship, instantaneous death rate and life expectancy. The mechanism involved in these effects is likely to be exploitation competition for especially profitable food items such as diatoms.