There has been isolated a Schistosoma mansoni strain from two patients submitted to two courses of treatment with hycanthone (2,5mg/kg, i.m.), in January and April, 1970, and to one course with niridazole (25mg/kg/day x 5, per os), in April, 1971. Before treatment, the number of eggs in the faeces of those patients was, per gram, 2,675 and 1,025, respectively; after completion of treatment, such number had come down to around 100 eggs/gram. Miracidia hatched from the patients faeces could infect Biomphalaria glabrata snails, which elimmated cercariae (WW strain) that were used for experimental infection of albino mice. The infected animais were, afterwards, treated with hycanthone, niridazole and oxamniquine under various schedules. Comparative studies of WW and LE strains (the latter being routinely kept in our laboratories) revealed marked differences in their sensitivity to the schistosomicides employed. Actually, after treatment with hycanthone, at the dosage of 80 mg/kg, i.m., a 100% oogram changes were observed in the intestinal wall of mice inoculated with LE strain, whereas no alterations could be detected in the mice infected with WW strain. As regards oxamniquine and niridazole the changes were smaller although still quite sufficient to indicate greater resistance of WW strain to these schistosomicides. It is worth while remembering that, in the pertaining literature, it is the first time that resistance in S. mansoni strains from treated patients has been demonstrated.