A survey of the prevalence of bilharziasis has been made in the Egypt-49 Project Area, part of the Beheira province of Egypt with an area of 422 km(2) and a population of nearly 250 000 in 552 villages. The area has been classified into four divisions-Rural (Agricultural), Urban (Industrial), Reclamation (Resettlement) and Control, and subdivided into 23 sections bounded by irrigation canals or drains. Between April 1962 and March 1963, 11 944 individuals from 2573 households in 96 villages were examined.An over-all 5% two-stage cluster sample was obtained in each division, except in the Reclamation Division, where, because of the sparse population, a 10% systematic sample was used. The age and sex distributions of the samples were representative of those of the population as a whole. Larger villages tended to be over-represented, but there is no evidence that bilharziasis prevalence is related to village size.Bilharziasis was diagnosed by the detection of eggs in faeces or urine; details of the procedures are given. Examination of single samples, as in this study, leads to an underestimate of the extent of infection, by about 20% for S. mansoni infections and about 13% for S. haematobium; this fact has to be borne in mind in interpreting the prevalence data obtained.