Hypophthalmus marginatus is a siluriforme, belonging to the family Pimelodidae. This species is highly consumed in eastern Amazonia and has a wide socio-economic importance to the region. Studies on the histological characterization of the reproductive system are still unknown in this species. Therefore, this study serves as a tool for biotechnology techniques, necessary for the production in captivity, since this species is one of the fishing resource most intensively exploited by artisanal fisheries. The aim of this study was to characterize the structural and ultrastructural features of germinal epithelium cells of Hypophthalmus marginatus during spermatogenesis. 168 specimens were collected from December 2011 to December 2012. Macroscopically were identified 69 adult males. In field, ventral incision was held in fishes and fragments of testicles from the median region and semen were removed. Then the material was subjected to processing and analysis of light microscopy, transmission electron microscopy and scanning. The data revealed that the germ cells proliferate inside the spermatic cyst synchronously. Spermatids enter at the spermiogenesis process through three distinct stages of cell differentiation. The nucleus does not move relative to the axis flagellar and the centrioles don't migrate to the nucleus. The spermatozoids showed a spherical head with the highly condensed nucleus and no acrosome. The intermediary piece was short containing a single long flagellum. The axoneme presents the structure of microtubules within the pattern (9 + 2). The nuclear fossa housed the proximal centriole partially. The presence of spermatogenic cells in the same subphase of meiosis inside the cyst is characterized as cystic spermatogenesis, with synchronous growth of the cells. This suggests that spermiogenesis of H. marginatus is of type III and the presence of nuclear fossa is considered a distinctive feature for the species, which differs from other pimelodids.
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