The characteristics of 163 spontaneous neoplasms diagnosed in 150 necropsied zoo mammals, birds, and reptiles at Taipei Zoo during 1994-2003 were analyzed. Histopathology and immunohistochemistry were employed to classify the tumor types. A total of 2657 necropsied zoo animals, including 1335 mammals, 873 birds and 449 reptiles led to the diagnosis of tumor in 8.1% (108/1335), 4.2% (37/873) and 1.1% (5/449) of cases, respectively. The most predominant type of tumors in mammals was mammary gland tumors (12.0%, 13/108), followed by uterine smooth muscle tumors (10.2%, 11/108), lymphosarcoma (9.3%, 10/108), hepatocellular carcinoma (8.3%, 9/108), and cutaneous squamous cell carcinoma (6.5%, 7/108). The avian neoplasm with the highest incidence was lymphosarcoma (35.1%, 13/37). Five individual neoplasms were found in different reptile species. The overall incidence of malignant tumors (63.8%, 104/163) was greater than that of benign tumors (36.2%, 59/163). Immunohistochemistry characterization of these tumors revealed a histogenesis which is similar to that seen in domestic animals and humans.