Abstract. Patients with schistosomiasis mansoni and schistosomiasis haematobium from the Gezira area of the Sudan were investigated for the simultaneous presence of Schistosoma mansoni and S. haematobium eggs in urine. Before treatment, 28 or 34 mixed-infection patients constantly excreted eggs of S. mansoni in the urine; however, the concentration was only 1.7% that of S. haematobium eggs. Patients were given two doses of metrifonate (10 mg/kg body weight) 2 weeks apart in order to compare the effect of the organophosphorous compound on the two parasite species. Each dose of metrifonate was followed by a significant decrease in egg output in urine (P < 0.01). The egg reduction was similar for both parasite species, and was almost 99% after the second treatment. Before treatment was started, a positive correlation existed between the numbers of S. haematobium and S. mansoni eggs excreted in urine (r = 0.75, P < 0.001), and this correlation did not change after the first or the second dose of metrifonate. After treatment, in 6 of 37 patients S. haematobium eggs and in 8 of 28 patients S. mansoni eggs were not, or were only slightly, reduced. When chemotherapeutic failure of metrifonate against S. haematobium or S. mansoni occurred, it was quantitatively similar for both parasite species.