Dichlorvos (dichlorovinyl dimethyl phosphoric acid ester) is a cholinesterase inhibitor used widely as a contact and stomach insecticide for control of internal and external parasites. Carcinogenesis studies were conducted by administering dichlorvos in corn oil by gavage 5 times a week for 103 weeks to groups of 50 male and 50 female Fischer rats at 0, 4, or 8 mg/kg body weight, to groups of 50 male B6C3F1 mice at 0, 10, or 20 mg/kg, and to groups of 50 female B6C3F1 mice at 0, 20, or 40 mg/kg. During the course of the studies, body weights and survival rates of the male and female rats and mice were not different from those of their respective controls; females of both species appeared to gain more weight than controls. Neoplasms induced by dichlorvos included adenomas of the exocrine pancreas (male rats), mononuclear cell leukemia (male rats), and squamous cell papilloma of the forestomach (male and female mice; two other female mice had squamous cell carcinomas). Lesions observed in female rats that may have been due to dichlorvos administration included adenomas of the exocrine pancreas and fibroadenomas of the mammary gland. The results demonstrated that dichlorvos is carcinogenic for Fischer rats and B6C3F1 mice.