Aflatoxin M1, a hydroxylated metabolite of aflatoxin B1, has been found in the milk, urine, and tissues of animals which have consumed aflatoxin B1. Aflatoxin M1, isolated from cultures of Aspergillus f1avus and fed to rainbow trout, was a potent liver carcinogen but less carcinogenic than aflatoxin B1. Levels of 4 and 16 parts per billion (ppb) aflatoxin M1 in a purified test diet produced a 13 and 60% incidence, respectively, of hepatoma in 12 months. Cyclopropenoid fatty acids (CPFA), which promote the carcinogenicity of aflatoxin B1 in trout, behave similarly with M1. Dietary levels of 4 ppb aflatoxin M1 with CPFA gave a 70% incidence of liver cancer in 8 months. Trout that received 20 ppb aflatoxin M1 in their diet for 5–30 days developed a 3–12% incidence of hepatoma in 12 months. A significant number of mortalities occurred amongfemale trout with aflatoxin M1-induced hepatomas at time of maturation (16–20 months) in contrast to no mortality in males.