SUMMARYIntensive studies of 208 single-ascospore mycelia ofHypomyces Ipomoeaehave shown each to be self-sterile and to fall into one or the other of two reaction groups. Members of each reaction- group are sterile when matedinter se,and react with only one of the two tester-strains bearing opposite sexual-reaction factors. All matings of members of one group with members of the other group have proved fertile.All single-spore mycelia produce sterile bodies which are with reason assumed to bear the female sex organs. Nearly all such bodies in compatible matings develop into fertile perithecia.Microconidia of members of either reaction-group appear to be capable of“spermatizing”the female sex organs of members of the other reaction-group.It is proposed that each single-ascospore mycelium is hermaphroditic, but self-sterile. Each monoploid nucleus must, then, possess the potentialities of both sexes. Hence, sexual-reaction factors, rather than sex factors or sex chromosomes as such, are segregated at meiosis. The data strongly suggest that a single allelomorphic pair is concerned.