Alternaria alternata is commonly found in the grain in areas of high incidence of oesophageal cancer and is a suspected cancer-causing factor in Linxian, China. In this study, this fungus was isolated from corn in Linxian and cultured. The extract of this fungus was shown to induce 6-thioguanine-resistant mutants in V79 cells and cause transformation of NIH/3T3 mouse fibroblast cells. Metabolic activation does not seem to be required for these activities. The mutagenic and transforming activities of the extract of A. alternata suggest that this fungus may be a factor in the etiology of oesophageal cancer in Linxian.