Sexual differentiation in male strains of the aquatic fungus Achlya ambisexualis Raper is induced by antheridiol, a sexual steroid hormone secreted by female strains. Antheridiol-induced initiation of the morphologically distinct antheridial branches in male Achlya is completely prevented when DNA-dependent RNA synthesis is inhibited by actinomycin D. In addition antheridial branch elongation is inhibited to a degree proportional to the concentration of actinomycin D added. Thus, evidence indicates that RNA synthesis is required for antheridiol-induced initiation of antheridial branching and that continued RNA synthesis is required for elongation of antheridial branches.