IN general, a heterocaryon between a white and a yellow mutant of Aspergillus fungi gives rise to white, yellow, and green progeny. It was found recently, however, that five heterocaryons produced between two of the seven colour mutants of A. sojae strain K could give brown progeny besides the white, yellow, and green. Evidence that the brown strain appears to be a heterocaryon possessing haploid and diploid nuclei in a common cytoplasm is to be described.