Seven strains of Schistosoma mansoni were isolated from the feces of patients. Three of the patients had been unsuccessfully treated with hycanthone or oxamniquine; the remaining four had not been given any specific treatment. Mice infected with these strains were treated 45 days later with niridazole (50 or 100 mg/kg body weight per day for 5 days, per os), hycanthone (single dose of 80 or 160 mg/kg, intramuscularly), or oxamniquine (single dose of 50 or 100 mg/kg, per os). Significant differences in the strains' responses to the schistosomicidal agents used were observed. Oogram changes in mice treated with the same drug ranged from 10–100%, and percentages of dead worms from 0–45. Such differences seemed to be dependent on the strain's specific characters rather than to their having been exposed to schistosomicidal drugs.