Gross examination of a 24-month-old, male cynomolgus monkey (Macaca fascicularis) revealed obstruction of the ileum by a mass that entrapped and compressed the ileocecal junction. The mass was well circumscribed, firm, and white on cut surface. Histologically, the mass consisted of spindle-shaped cells arranged in interweaving bundles or as narrow cords and individual cells widely separated by dense collagen. A diagnosis of localized retroperitoneal fibromatosis was made based on the characteristic gross and microscopic findings and isolation of type D simian retrovirus, serotype-2, from spleen and mesenteric lymph node. Monkeys with localized retroperitoneal fibromatosis generally exhibit signs only of a palpable mass at the ileocecal junction and/or nonspecific diarrhea. This case represents an unusual presentation of localized retroperitoneal fibromatosis in which the lesion produced intestinal obstruction and death.