A longitudinal study has been conducted (1991-1997) to evaluate the impact of repeated selective chemotherapy on human transmission indices of Schistosoma mansoni infection. The study population included 8370 individuals inhabiting four villages and their satellites and representing high and low S. mansoni endemicity communities in Kafr EI-Sheikh governorate. A parasitological survey was conducted for three successive years (1991-1992 and 1993). Each time infected received praziquantel (PZQ) chemotherapy. In 1997, a fourth parasitological survey was done. During the period from 1994-1996 only routine diagnosis and treatment of schistosomiasis offered by the Ministry of Health and Population (MOHP) was going on. Study results revealed a decrease in S. mansoni prevalence and intensity of infection in the first three years. The drop was marked after the first year. In 1997, after the cessation of active case finding and treatment by the project team, an upward trend was observed for both prevalence and geometric mean egg count (GMEC) especially for the high prevalence villages. However, all indices were kept at significant low levels as compared to base-line values in 1991.