Abstract
Schistosoma mansoni developing in mouse hosts subjected to frequent, heavy doses of stilbestrol show five effects: their normal survival rate is much reduced; the parasites for the most part remain in the liver past the time at which they normally migrate to mesenteric veins; both sexes develop to sexual maturity more slowly, if at all; a high proportion of the males have accessory gonadal tissue which probably is mainly ovarian; and the male worms average less than two-thirds as long as controls.