Evidence was found for a diffusible sexual hormone controlling the development of the sexual organs in Dictyuchus monosporus. Since zoospores which might be responsible for the sexual reaction were shown to be passing from one beaker to the next in perfusion experiments in which a small stream of water was passed over plants of both sexes, a Millipore filtering system was devised to restrain the zoospores but allow all of the water in which the mating strains were being crossed to flow over the test culture. It was only when the filtrate came from crosses between male and female strains and when the test culture was female that any reaction occurred. The sequence of action of the hormone was not determined.