Aims This study was carried up to assess the impact of mass chemotherapy with praziquantel on the prevalence and intensity of Schistosoma haematobium and Schistosoma mansoni in Office du Niger. Materials and Methods It was a cross sectional study with two passages about which we compared a test group of 7 villages (n=2342) treated in 1989 to a control group of 7 villages (n=2263). Results One year after mass chemotherapy, baseline prevalence rates of Schistoosma haematobium, Schistoaoma mansoni and those of the double infection decreased by 50%, 54% and 62,9% respectively. The geometric mean egg counts of Schistosma haematobium (GMECSh) was reduced by 66.6% and that of Schistosoma mansoni (GMECSm) by 43.4%. However, in spite of mass treatment, the overall prevalence rates of Schistosoma haematobium and mansoni were always higher than 20% in young people aged of 6-19 years. Conclusion These data show that in irrigated area, efficacy of praziquantel is strongly affected by age. This persistance of infection in population raises up several questions according to host immunity, parasite biology and praziquantel efficacy.