The population dynamics of the freshwater snail Biomphalaria pfeifferi, an intermediate host of the parasitic worm Schistosoma mansoni, was studied in the Virunga stream in Eastern Zaïre. Methods are developed to estimate age-specific survivorship from regular quadrat-sampling data without assuming a constant mortality rate, and to approximate age-specific fecundity when the fecundity function is known in the laboratory. The population dynamics in the field was found to be very different from that in the laboratory: it is basically discontinuous, with one main generation per year; a massive mortality occurs after hatching; fecundity is reduced considerably; and there are marked seasonal variations in both survivorship and fecundity. Time-dependent demographic parameters were estimated by month instead of by cohort. They show that the environmental conditions are favourable to an increase of the population only during a short period of the year. The intrinsic rate of natural increase, r, was found to be well correlated with the changes in egg density. The environmental factors determining the population dynamics in the Virunga are discussed; current speed seems critical.