Laboratory mice were infected with S. mansoni. then given 26 mg of crystalline testosterone over a period of 49 days. Eggs were collected at that time, and the 2d generation of worms obtained. 1341 worms were recovered, of which 667 were males, 434 females, and 240 male-like hermaphrodites. The sex ratio among 2643 worms from normal mouse hosts was 1321 males, 1314 females, and 8 male-like hermaphrodites. It seems likely that the testosterone induced the imbalance of sexes and appearance of hermaphrodites in the 2d generation of experimental worms. The experiment is being repeated in order to verify this effect, and to find during what period of the worms'' development the testosterone is most effective.