Males of Myleus ternetzi, at various maturation stages were netted in a French Guyana river. Testes were histologically and ultrastructurally analysed. Leydig cells were always present in large numbers. In mature or almost mature testes, they were characterized by a large spherical nucleus and an extensively developed smooth endoplasmic reticulum (SER) composed of very narrow tightly packed tubules. In regressed testes, most of the Leydig cells had a smaller nucleus with an irregular shape, a disorganized SER and the fine structure of the mitochondria was greatly affected. These changes would reflect different phases throughout a metabolic cycle in relation to spermatogenesis.