Lake Valencia is located in the centre of the endemic area of the intestinal schistosimiasis in Venezuela. The dominance of two pulmonate species, Biomphalaria glabrata and B. prona., was observed in the lake. Both species are strongly associated with two distinct types of habitats suggesting that competition is occurring between these two species. B. glabrata and B. prona play the role of intermediate hosts of schistosomes in Venezuela. At the present time, parasite transmission is not occurring in the lake but the planning of important development programmes represents a risk of creation of active schistosomiasis foci. The knowledge of the importance and distribution of the snail host populations is therefore essential and must be taken into account for developing future control strategies.