A spontaneous case of external endometriosis in a 22-year-old female De Brazza's monkey is described. During diagnostic laparotomy, peritoneal adhesions of the uterus to the urinary bladder, ovaries and abdominal wall were seen. A surgical biopsy of soft tissue attached to the ventral surface of the uterus was examined histologically and consisted of endometrial glands and stroma. These findings were compatible with a diagnosis of endometriosis.