The author discusses the relation between irrigation systems and the transmission of bilharziasis, with special reference to the important part the irrigation engineer can play in checking the spread of the disease. He points out that, in the past, there has been little co-operation between health departments and public works agencies in respect of the setting-up of irrigation systems, and stresses the advantages to be gained from an active collaboration between malacologists, epidemiologists and irrigation engineers at the planning stage of irrigation schemes.The author also puts forward some suggestions for research on irrigation-system design and outlines the role of WHO in bilharziasis control.