Histological structure of the testes and development of spermatozoa in Jenynsia species is described using light, scanning and transmission electron microscopy. The testis type is restricted spermatogonial, wherein spermatogonia are restricted to the distal ends of lobules, typical of the Atherinomorpha, and spermatogenesis is continuous throughout the year in wild-caught fish. Within the testicular lobes there are lobular germinal compartments wherein the functional units are spermatocysts, whose borders are formed by Sertoli cells. Spermatocysts may contain meiotic primary spermatocytes, secondary spermatocytes, spermatids, undergoing spermiogenesis, or spermatozoa. Spermatocysts with later stages of developing sperm are located proximal to the testicular ducts. During spermiogenesis, spermatid nuclei become elongated. As this occurs, the nucleus develops a deep, central fossa that contains the centriolar complex. As the flagellum grows, enlarging spermatid mitochondria migrate posteriorly alongside the flagellum but remain separated from it by the cytoplasmatic canal, an indentation of the plasma membrane. Between the enlarged mitochondria and plasma membrane, a sub-mitochondrial net develops. In longitudinal sections, the enlarged mitochondria are stacked in a zig-zag fashion, and in transverse sections they appear as a ring surrounding the flagellum, but separated from it by the cytoplasmic canal. Spermatozoa of the 3 jenynsiid species have an introsperm complex composed of a long mid-piece whose flagellum has a single "wing." Within the efferent ducts and the tubular gonopodium, sperm are lightly packed in a side by side fashion which facilitates their transfer into the female reproductive tract. This study presents detailed descriptions of testicular organization and cytological characterization of the stages of spermatozoa differentiation in 3 species of Jenynsia from northwestern Argentina (J. alternimaculata, J. multidentata and J. maculata), in order to contribute to the understanding of testicular structure and development of spermatozoa in the context of evolution of viviparity in this fish lineage.