SUMMARYParasexual crosses involving a mutant ofAspergillus flavushaving a high and relatively stable level of aflatoxin B2relative to aflatoxin B1have revealed a genetic locus for control of aflatoxin synthesis. Five different diploids, each heterozygous for high aflatoxin B2levels, accumulated low amounts of B2but high amounts of B1. Although the haploid segregants from these diploids included a variety of aflatoxin phenotypes, one-half produced more B1than B1and one-half produced less B2than B1. Five different diploids, each homozygous for high aflatoxin B2levels accumulated high amounts of B2as did all haploid segregants from these diploids. A single genetic locus is thought to control the higher aflatoxin B2than B1accumulation in this mutant.