The main characteristic features of spermiogenesis in Chilodus punctatus (Characiformes) are rotation of the nucleus, development of a nuclear fossa, which extends as a narrow invagination deep into the nucleus and the way in which flagellum is formed. The chromatin condensation proceeds during the spermiogenesis from heterogeneous through homogenous and granular to a highly compact one present in the mature spermatozoon. Mature Ch. punctatus spermatozoon shows a spherical nucleus, short midpiece and flagellum with lateral fins. The centrioles are in perpendicular arrangement and are located in the deep nuclear fossa, which extends towards the anterior pole of the nucleus. The midpiece contains a few mitochondria, which are separated from the anterior fragment of flagellum by the cytoplasmic channel. Spermiogenesis and spermatozoon ultrastructure conform to the pattern observed in other ostariophysans, but for the first time the presence of lateral fins along flagellum has been documented in a representative of Characiformes.