Buffered mycelial extracts were prepared from 20 isolates ofAspergillus flavusLink ex. Fries and four isolates ofAspergillus parasitionsSpeare, including aflatoxin-producing and non-producing strains within each species. The extracts were tested for activity of various enzymes after separation by polyacrylamide gel-slab electrophoresis.Aspergillusspp. were most readily distinguished by the bands of α-esterase (EC 3.1.1.2), β-esterase (EC 3.1.1.1), and peroxidase (EC 1.11.1.7). For each isolate, the relative mobilities were determined for the various multiple-molecular forms of all three enzymes. The isolates were then compared on the basis of these relative mobilities by using a simple index of similarity to construct a single-linkage dendrogram. No distinct differences in enzyme patterns were detected between aflatoxin-producing and non-producing isolates. However, patterns of similarity or groups were indicated in the dendrogram which suggested some degree of physiological relationship.