The cynomolgus monkey (Macaca fascicularis) is widely used in regulatory toxicity studies. Especially in studies on male contraception, the male reproductive tract can be an important target system. The aim of the present paper is to describe a practical approach for morphological staging of spermatogenesis in routinely prepared paraffin sections. Results obtained using this approach could help to determine possible drug-related effects on spermatogenesis. As a guide to the investigators, photomicrographs of Bouin-fixed, paraffin-embedded and H&E or PAS stained sections from testis tissue are presented to illustrate the twelve successive morphological stages (cell associations) of normal spermatogenesis. Sexually immature or peripubertal monkeys sometimes are included in toxicity studies. Since the morphological features of the testes of such monkeys can be mistaken for treatment-related abnormalities, the morphologic characteristics of these testes are described and discussed briefly.