In order to assess the efficacy of praziquantel against Schistosoma mansoni infection, a clinical trial was carried out in Namool village in patients with different intensities of infection according to the geometric mean egg count. Different doses were given (20, 30, 40 mg/kg body weight). A parasitological follow-up was carried out after 1 and 6 months from treatment. The percentage of cure was high and reached 89.4% among the low-egg-count group who received 40 mg/kg body weight. A high percentage of reduction was noted in the mean egg count of the non-cured cases which reached 92.6% in the high-egg-count group who received 40 mg/kg body weight of praziquantel. Mild side effects increased in frequency as dosage increased, abdominal disturbance, vomiting and fever. Praziquantel seems to be an ideal drug against Schistosoma mansoni infection especially for mass treatment as it is given in a single oral dose.