The Reinke crystal is a distinctive feature of human Leydig cells of the adult maturation phase. This large subcellular structure, first described in 1896 by Reinke, has been well documented ultrastructurally by numerous investigators during the past few decades (e.g. Fawcett & Burgos, 1960; de Kretser, 1967; Sohval et al. 1973; Christensen, 1975; Kerr, 1991). Although Leydig cells of many mammals contain unusual inclusions, the Reinke crystal has long been considered unique to the human Leydig cell. Its precise molecular composition and significance are unknown. It is, however, known to contain proteins (Janko & Sandberg, 1970) and has been found to increase within the cell with age (Mori et al. 1978). The purpose of the present report is to describe the presence of an inclusion with much similarity to the Reinke crystal within Leydig cells of the marmoset monkey (Callithrix jacchus).