RNAs from three Aspergillus parasiticus and three Aspergillus sojae isolates were probed with seven genes involved in aflatoxin biosynthesis. Previously published work and preliminary work in this study demonstrated that these aflatoxin biosynthesis genes were present in the DNA of the isolates. RNA from aflatoxin-producing and O-methylsterigmatocystin-producing A. parasiticus strains SRRC 143 and SRRC 2043 hybridized to all of the gene probes tested. However, RNA from a strain of A. palasiticus that had lost its ability to produce aflatoxin in culture (SRRC 77) and RNA from one of the A. sojae isolates did not hybridize to any of the gene probes. Two of the A. sojae isolates hybridized to the regulatory gene aflR and the structual gene uvm8, which is believed to code for a fatty acid synthase involved in an early step in aflatoxin biosynthesis, but not to any of the other five genes of the aflatoxin pathway tested. These results suggest that most of the genes involved in aflatoxin production are transcriptionally blocked in A. parasiticus SRRC 77 and all of the A. sojae isolates. The cause of this blockage is unknown.