Eight cases of acinar cell carcinoma of the exocrine pancreas were diagnosed in Japanese medaka (Oryzias latipes) from a pool of approximately 10,000 specimens used in a variety of carcinogenesis bioassays. Three cases occurred in specimens from control groups and one case in a specimen exposed to benzo(e)pyrene (BeP), a noncarcinogenic isomer of benzo(a)pyrene (BaP). Four cases occurred in specimens exposed to known carcinogens including two cases in specimens exposed to 7,12-dimethylbenz(a)anthracene (DMBA), one in a medaka exposed to benzo(a)pyrene, and one in a specimen exposed to methylazoxymethanol acetate (MAM-Ac). Pancreatic carcinoma was probably the cause of death in five cases. The neoplasms appeared to originate in the mesentery near the abdominal surface of the liver.