Species of the genus Aspergillus, mainly A. flavus strains, occuring in the Araraquarense region (State of São Paulo, Brazil), were isolated and their ability to produce aflatoxin was studied. The isolation was conducted following routine methods in microbiology. The cultures were obtained from samples of peanuts collected in the 'rainy season" and in the "dry season". After the isolation the cultures were classified as follows: 102 A. flavus; 4 A. oryzae var. effusus; 2 A. oryzae; 2 A. parasiticus and 1 A. ochraceus. During the work, 4 cultures were lost, remaining 107 cultures. The cultures were then tested in their ability to produce aflatoxin, which was extracted from the mycelium and from the culture medium. The metabolites were separated by thin layer chromatography using the solvent benzene-ethyl acetate-ethanol and quantified under long wave U. V. light. From 107 cultures of Aspergillus only 33 (31%) produced aflatoxin; seven of those produced it only in the culture medium, while two, only in the mycelium. The production of aflatoxin was much higher in the mycelium than in the culture medium; some of them produced very high quantities, up to 400 p.p.m. of B1 and 300 p.p.m. of G1. From the 33 cultures producing aflatoxin, 32 belonged to the A. flavus species and only one to the A. oryzae. Previous studies when high yields of aflatoxin G1 as related to B1 were reported in that region, were confirmed in the present work.