Bulinus (B.) truncatus rohlfsi is an important snail host of Schistosoma haematobium in Ghana and probably elsewhere in West Africa. Study of this snail in natural habitats in Northern Ghana has shown that the pronounced population fluctuations can be broadly related to the alternating wet and dry seasons and to any marked changes in the aquatic vegetation. An increase in snail density and reproductive activity begins during the rainy season, reaching a peak in the dry season. The onset of the contraction phase in the snail population is often abrupt, although it may be preceded by intense oviposition; during this phase there are but a few widely scattered snails and little reproductive activity with a low level of survival, particularly of juvenile snails. The factors which favour the survival of young specimens are clearly critical in the evolution of the snail population.The findings are related to snail population studies carried out elsewhere in Africa, and to the application as well as limitations of molluscicides in bilharziasis control programmes.