To determine whether the sharply declining Schistosoma haematobium infection rates in parts of the Nile Delta could be generalized to the entire region, and to update the status of S. mansoni infection rates, a large scale survey was undertaken in 1983 in 70 of the 71 districts of the Nile Delta. In a house-to-house survey, > 91% of the sample population of 16,675 participated by providing stool and/or urine specimens which were examined qualitatively by Kato thick smear and sedimentation techniques, respectively. After the 1935 survey by Scott, the prevalence of S. mansoni appeared to change little, from 33% in 1935 to 39% in 1983, but a more sensitive diagnostic technique in 1983 strongly suggested that the actual prevalence had decreased between the 2 surveys. In contrast, the prevalence of S. haematobium infection decreased from 56% to 5%, with a similar decline in all 8 governorates. The dramatic decline in S. haematobium prevalence has been accompanied temporally with a sharp decrease in the population density of Bulinus truncatus. S. mansoni has become the predominant human schistosome species in the Nile Delta.