A method has been developed for the production of aflatoxin by growing Aspergillus flavus strain NRRL 2999 on the solid substrate rice. Optimal yields, more than 1 mg of aflatoxin B 1 per g of starting material, were obtained in 5 days at 28 C. A crude product containing aflatoxins was isolated by chloroform extraction and precipitation with hexane from concentrated solutions. The crude product consisted of 50% aflatoxin in the following ratio: B 1 -B 2 -G 1 -G 2 , 100:0.15:0.22:0.02. Aflatoxin B 1 was separated from almost all the impurities and from the other aflatoxins by chromatography on silica gel with 1% ethyl alcohol in chloroform. Analytically pure aflatoxin B 1 was recrystallized from chloroform-hexane mixtures.