A microscopically unusual, malignant neoplasm of the stomach that had developed spontaneously in an old female Rhesus monkey is reported. It was concluded that this tumor represented a carcinoma in which highly anaplastic cellular elements were intermixed with well-differentiated adenocarcinoma cells. A great deal of stromal proliferation was also observed in many areas of the tumor, such as is characteristic for a scirrhous carcinoma. The neoplasm had invaded the stomach wall and the tumor cells showed all criteria of malignancy, but metastases were not found, which is unusual for a tumor this size. It was felt that the neoplasm had developed spontaneously since there is no evidence that exposures to poliomyelitis or pesticides influenced the development of cancer in this monkey.