Colon cancer is the second most common cause of cancer mortality in human beings, but relatively few cases have been described in macaques. The present report documents two fatal cases of colon cancer in aged rhesus macaques. Case 1 was a 20-year old female in which extensive invasion of the caecum by a scirrhous adenocarcinoma led to perforation and a severe fibrinopurulent peritonitis. Case 2 was a 32-year old male with a stricture at the ileocaecal junction, also caused by a scirrhous adenocarcinoma, which had metastasized to a regional lymph node. Both neoplasms showed aggressive local involvement of the proximal large bowel, which appears to be a predilection site in rhesus monkeys. Descriptions of spontaneous cases of colon cancer in non-human primates may lead to the development of models for certain aspects of the disease in man.