Changes in in vivo polypeptide synthetic patterns during sexual morphogenesis of Achlya ambisexualis E87 were investigated using polyacrylamide gel electrophoresis. Autoradiography of [35S]methionine-labelled polypeptides revealed the apparent qualitative synthesis of a 64 000 dalton polypeptide, commencing 30–60 min after antheridiol addition and continuing for at least 6 h. Synthesis of this pheromone-inducible polypeptide was detected throughout male sexual morphogenesis in still culture matings of the male strain E87 and the female strain 734. Two other species of Achlya were also found to synthesize the 64 000 dalton polypeptide in response to antheridiol. The inducible polypeptide did not appear to be associated strictly with a branching response, as no detectable synthesis occurred when asexual branching was induced by the exogenous addition of either casein hydrolysate or lactalbumin hydrolysate. Two-dimensional gel electrophoretic analysis revealed quantitative changes in the synthesis of many other polypeptides as a result of antheridiol action. In addition, both equilibrium and nonequilibrium two-dimensional gel electrophoresis showed the inducible polypeptide to be basic, demonstrating an isoelectric point of more than 8. Studies using the RNA synthetic inhibitor actinomycin D suggested a transcriptional control of the inducible polypeptide.