A 12-year-old, male black and white colobus monkey (Colobus guereza kikuyuensis) from a small community zoo presented with a 6-month history of mild, slowly progressive ataxia and paresis culminating in an acute episode of recumbency, depression, and seizures. The animal was humanely euthanatized. Gross post-mortem examination revealed significant abnormalities including diffuse pallor of the carcass and a firm, pale, 8-cm diameter mass, adherent to the serosa of the proximal duodenum and colon, and embedded within the pancreas and mesenteric root. Histologically, the mass had characteristics of a neuroendocrine or endocrine tumor. Immunohistochemical stains for chromogranin, synaptophysin, insulin, and glucagon were positive, confirming the diagnosis of a mixed pancreatic islet cell tumor. These tumors are rare in all species except ferrets and unreported previously in colobus monkeys.