Bulinus truncatus, the intermediate snail host ofSchistosoma haematobium, was widely distributed within the irrigation system at Tessaout-Amont area, located about 70 Km northeast of Marrakech. The lakes and the primary canals were not colonized, but secondary canals and especially various associated structures (syphon boxes, drop structures and weirs) were important habitats forB. truncatus. Bulinus truncatusis particularly abundant in tertiary canals, but it is there almost completely restricted to the syphon boxes as these canals, except for syphon boxes, dry out quickly when irrigation stops. The distribution ofB. truncatusseems to be governed by irrigation practices in terns of water management, rather than by water physicochemical parameters or occurrence of aquatic plants.