A convenient procedure consisting of UV photography (K. Yabe, Y. Ando, M. Ito, and N. Terakado, Appl. Environ. Microbiol. 53:230-234, 1987) and a tip culture method has been devised for the isolation and characterization of Aspergillus parasiticus mutants relating to aflatoxin production. With the latter procedure, the production of aflatoxins excreted into the culture medium and precursors in the mycelium were easily measured quantitatively or semiquantitatively. A total of 38 mutants in which the aflatoxigenicity was decreased or lost were obtained by UV radiation; 3 were found to be blocked mutants, which accumulated the aflatoxin precursors versicolorin A or averantin.