A survey of schistosomiasis mansoni infection was conducted in an area of northwestern Ethiopia which has been targeted for an irrigation project. The proximity of settlements to water-bodies infested with Biomphalaria pfeifferi, and used extensively for bathing and domestic purposes, indicated that the potential for transmission is high. Snails shedding S. mansoni cercariae were found in two localities. Stool specimens from a total of 7517 persons were examined, including specimens from 4915 children age 7-14 years in 29 localities, as well as from the total population of four of the more heavily infected localities. The 10-14 year age group had the highest transmission potential, in terms of prevalence (overall 25%), intensity of infection, and absolute numbers of infected individuals, suggesting that screening and chemotherapy would be most cost-effective in that age group.