Mature, egg-producing female worms from bisexual cercarial infections in mice were transplanted surgically either without male worms, to produce unisexual infections in recipient hamsters, or with male partners to produce bisexual infections. The morphology of the female reproductive organs and oogenesis in the unisexually and bisexually transplanted females were compared over a 9-day period. Females which paired with male worms in hamsters continued to produce eggs throughout the experiment. In contrast, unisexually transplanted females exhibited degeneration of the vitellaria at 3 days and the ovary at 6 days posttransplantation, although these worms produced fertilized oocytes as late as 9 days following transfer. Female worms which had degenerated upon separation from male worms for 3 mo regenerated and produced viable eggs when reunited with mature males. Thus, separation of females from their male partners leads to a reversible degeneration of the female reproductive tract.