The structure of chromatin during spermiogenesis of a lizard was examined. The fibrillar nature of chromatin is maintained during all time even in the mature sperm, but the fibers pass through a process of thickening and reorganization which begins with a homogeneous and diffuse aspect, going through an organization of chromatin fibers parallel to the long axis of the nucleus, disposed concentrically but not evenly displaced. In a final stage the fibers are thicker and closely packed showing almost no nucleoplasm among them.