The results of a 5-year study on the distribution and ecology of Bulinus truncatus in Khuzestan, south-western Iran, are described. Live snails were found in 4 foci, namely, the main focus, the upper Karun River, the lower Karkheh River, and the Bala Rud watershed; in addition, dead snails were found in 3 other foci. The patchy distribution of snails was related to the presence of surface waters and to their chemical composition.Monthly observations on the population dynamics of the snails and on the role of the snails in the transmission of Schistosoma haematobium and S. bovis were carried out in 14 ponds, 9 canals, 5 swamps, 2 field drains, 1 spring and 1 stream-pool. In standing waters, the peak of snail population could potentially occur in 2 seasons-May-July and November-January-but the main seasons for the transmission of mammalian schistosome cercariae were April-May and October-November. In flowing waters, a higher number of snails might be found in summer in some canals and in winter in another group of canals. Canals were found to be more important sites of transmission of human schistosomes than were village ponds. Transmission took place mainly in hot months when man-water contacts were greatest. Village ponds accessible to livestock were found to be important in the transmission of bovine schistosomiasis. As new irrigation systems have been expanding in this area of Khuzestan, more attention should be paid to flowing waters than to standing waters in future schistosomiasis control programmes.