Forty-one fungal isolates (one isolate per species) representing common plant pathogens and food crop contaminants were grown on sterile, polished rice and assayed for mutagenic activity in the Salmonella typhimurium-microsome system. Initially, single doses of aqueous and chloroform extracts of the moldy rice were assayed against the TA100 tester strain by incorporating extracts into the growth medium and by applying small quantities on disks placed on the agar surface. Suspected activity was examined further by analysis of several doses in the plate incorporation assay. Extracts of two aflatoxin-producing isolates (Aspergillus flavus and A. parasiticus) showed pronounced mutagenic activity, as did extracts of five other isolates (A. heterothallicus, A. nidulans, A. terricola, Alternaria tenuis, and Fusarium moniliforme) which did not contain detectable aflatoxins. Seven additional isolates (Botrytis cineria, Ceratocystis fimbriata, Cladosporium herbarum, Fusarium solani f. sp. pisi, Penicillium oxalicum, Thermomyces lanuginosus, and Verticilium albo-atrum) revealed activity which was possibly mutagenic; i.e., mutagenic responses were not observed in both the disk and incorporation assays, and clear dose-related activity was not observed in the incorporation assay. Extracts of the remaining fungi were not mutagenic in the bacterial assay.