The steroid hormone antheridiol has previously been shown to play a number of key roles in regulating development of the male sex organs, antheridia, of the water moldAchlya ambisexualis. We now demonstrate that synthetic antheridiol can inhibit both sexual (determined by oogonial counts) and asexual reproduction (determined by counts of asexual spores) in an isolate of the homothallic speciesA. heterosexualis. The implications of this dual role of antheridiol as hormone and allomone are discussed.