The schistosomicidal. activities of seven 2-(alkytamino)-1-phenyl-1-ethanethiosulfuric acids (1a-g) (R = propyl, isopropyl, butyl, isobutyl, t-butyl, sec-butyl, cyclohexyl, respectively) were determined in female Swiss mice infected with Schistosoma mansoni. The compounds were administered in a single oral dose of 800 mg/kg to groups of 15 mice infected with 50 cercariae each. All the compounds were found to be active, a high animal mortality being observed with 1e. These compounds have a high specificity against female worms (64-100% reduction vs. 33-61% reduction in mate worms). The test was repeated, a 400-mg/kg sub-dose of 1f also being tested, and similar results were observed. A 94% reduction in the number of female worms was observed when compound 1c was administered in a single 800-mg/kg dose to animals infected with 80 cercariae. Finally, the test was repeated with single 800 mg/kg oral doses of compounds 1e (highly purified) and 1f and a 400-mg/kg sub-dose of 1c. The toxicity of le was confirmed, while the animals that received 1c and 1f presented reductions in the worm Loads corresponding to 45.9% (male worms) and 84.8% (female worms) for 1c and 50.4% (male worms) and 94.2% (female worms) for 1f.