Sixteen predominantly male Mendelian mutants were isolated. All belong to one of the four linkage groups of predominantly male mutants previously described (mal-1, mal-2, mal3 and mal-4). Predominantly female mutants (f-i and f-3) segregating in a Mendelian fashion were also isolated. The male mutants mal-2, mal-3 and mal-4 are epistatic tof-i andf-3, as double mutants (f mal) are predominantly male. In contrast, f-1 mal-l double mutants are predominantly female, while f-3 mal-1 double mutants are wild type-like, or have an excess of male gametangia. Strains with f-l and f-3 develop up to 50% male gametangia when the incubation temperature is lowered from the normal 35 C to 23 C. Suppressor mutations (sphenotypically wild type-like) of mal-1 were also found, and s mal-1 double mutants are hermaphroditic as is the wild type. In the subgenus Euallomyces (Emerson, 1941) (Chytridiomycetes, Blastocladiales) there is an alternation of isomorphic, mycelial generations. The sporophyte produces colorless zoosporangia releasing zoospores which perpetuate the sporophytic generation, and brown, drought resistant resting sporangia (meiosporangia). After meiosis the resting sporangia release meiospores which following encystment develop into the gametophytic generation. The gametophyte is hermaphroditic and homothallic and bears chains of alternating small, orange male gametangia and large, colorless female gametangia. The sex of the terminal gametangium is species specific. In Allomyces macrogynus (Emerson) Emerson and Wilson the terminal gametangium is male, whereas A. arbuscula Butler has a female gametangium in the terminal position. The sexual differentiation in Allomyces is determined by nuclear as well as mitochondrial genes. Non-Mendelian male mutants of A. macrogynus were isolated by R0nne and Olson (1976) and Olson et al. (1982) after treatment with acridine dyes. The only reported isolation of predominantly female mutants of Allomyces was made by Foley (1958). She isolated spontaneous as well as UVinduced female strains of Allomyces arbuscula. These mutants additionally had a poorly defined nutritional requirement and segregated in a non-Mendelian fashion. Stumm (1958) isolated a pure male mutant (BM-8) of Allomyces arbuscula after X-ray treatment of meiospores, and found that the male phenotype segregated in a Mendelian fashion. As shown by Olson and Borkhardt (1978) and Borkhardt and Olson (1979), A. macrogynus strain Burma 3-35 (35?C) is haploid as a gametophyte, while natural isolates are almost exclusively diploid gametophytes (Emerson and Wilson, 1954). A number of mutants have been isolated from A. macrogynus strain Burma 3-35 (35?C). Olson and Nielsen (1981) and Nielsen and Olson (1982) isolated and described a number of X-ray-induced auxotrophic mutants, and after X-ray treatment of meiospores Olson et al. (1982) isolated predominantly male mutants which segregated in a Mendelian fashion, indicating that they were of nuclear origin. Olson et al. (1982) showed that seven Mendelian predominantly male mutants belonged to four different linkage groups. This seemed to indicate that the number of nuclear genes involved in the determination of maleness is rather