A new dioecious sp. of Achlya, A. heterosexualis , is described. It differs from the dioecious sp., A. bisexualis Coker et A. Couch and A. ambisexualis Raper, in undergoing self-conjuga-tion more frequently, the self-induced antheridial branches being monoclinous and androgynous in origin in A. heterosexualis and diclinous in the other 2 sp. The members of A. heterosexualis secrete less hormone A than those of A. bisexualis and A. ambisexualis. Cross-conjugation occurs between the members of A. heterosexualis and between the members of this sp. and the other dioecious sp.