Abstract
Several cross tests were carried out between a white methionineless mutant of Aspergillus oryzae and a yellow leucineless mutant of Asp. sojae. The mixed plating of two kinds of conidia onto a limitedly supplemented medium yielded brown prototrophic colonies in the frequency of 2-8 per 108 plated conidia. The new strains were quite stable even after sequential stransfers and partially equipped with hybrid characters. Four possible mechanisms were discussed in regard to the appearance of new strains.
