Three atypical strains (B-1, B-2 and C strains) of Aspergillus flavus were isolated in maize field soils. The B-1 strain which formed abundant microsclerotia and showed high aflatoxin B₁ producing ability was detected from about half of soil samples examined. The isolation frequency of it in the southern region was lower than that in other regions of Thailand. The B-2 and C strains produced B as well as G groups of aflatoxins. The distribution of the latter two strains in soils seemed to be limited.