Social and religious water-contact activities have been recorded over a period of 1 year in 4 villages situated on different types of watercourse in the Egypt-49 project area. Females have more frequent water contacts than males; females under 25 years old have more frequent contacts than older ones. Males are more active contaminators than females, among whom girls under 5 years old are the chief polluters. Summer is the period of most frequent, prolonged and extensive body- surface contact. One-quarter of all exposures take place between noon and 3 p.m., the period of maximum cercarial load. The most frequent activities during this critical period are washing of utensils, bathing and playing, and wadu (ritual washing). Peak exposures therefore coincide with the peak seasonal and diurnal cycle of infectivity of watercourses in the area. It is suggested that an index might be established that would take into account the frequency and duration of exposure and the amount of body-surface exposed and that could be related to the periodic variations of cercarial densities in the watercourses.