This study, as a part of the drug resistance project, one of the activities of the schistosomiasis research project, was carried out in 3 villages in lower Egypt. Its main objective is to assess the praziquantel cure rate and to identify infected persons not responding to praziquantel treatment. Among the studied population, the prevalence of S. mansoni among school children (age 6 to 18 years) was 21.8%, 32.7% and 32.4% in the 3 villages respectively. Those who were found infected were treated with praziquantel (40 mg/kg body weight). The cure rate after six weeks was 75.8%, 83.9% and 72.7% among school children respectively. This cure rate among school children was less than that of adults by 5-10%. Compared with that of preschool age, it showed variable results. A second dose after 6-8 weeks was given to those who were still infected. Those who remained infected even after this second treatment, received a third dose of 60 mg/kg body weight. After the third treatment, only 5 patients of school age remained infected compared to one among adults and one among preschool age. The failure rate was found to be 50%, 18.5% and 12.5% among preschool, school children and adults respectively after the third treatment regimen.