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 wasgreatly affected. These changes would reflect different phases throughout a metabolic cycle in relation to spermatogenesis.