Monoconidial and monoascosporic isolates of C. heterostrophus exhibit marked differences in degree of compatibility. The most critical comparison is made by determining the relative frequency with which isolates fail to cross with known compatible isolates. Studies of ascospore progeny of conidial crosses indicate that the differences in degree of compatibility are gene controlled. The data suggest that the genes controlling the differences in inter-group fertility influence the major gene locus for compatibility and give added support to the postulation that the sexual mechanism in the species is complex.