The main purpose of this study was to describe the morphology of the sperm packets produced by species in the genus Xenurobrycon. Males of the three species analyzed, X. macropus, X. polyancistrus, and X. heterodon, produce unencapsulated sperm bundles (spermatozeugmata) that are essentially identical in morphology. In longitudinal histological sections, each spermatozeugma is feather-like, with the tips of the sperm heads associated with a core that stains with the periodic acid-Schiff reagent (PAS ). As revealed by SEM, sperm heads are exposed only at the anterior end of the packet. Flagella cover the remainder of the packet and form a “tail” at the posterior end. The flagellum probably exits from a cytoplasmic collar at one side of the flattened sperm head, which tapers posteriorly. Spermatozeugmata are formed within the spermatocysts and only complete packets are released into the sperm ducts. Very few loose spermatozoa are seen within these ducts, attesting to the high degree of compactness of these sperm bundles. Insemination is confirmed in a specimen of X. macropus, whose ovary contained spermatozeugmata, demonstrating that intact packets can be transferred to the ovary by the male. A lateral flagellum, which is present in the xenurobryconin genera Scopaeocharax, Tyttocharax, and Xenurobrycon, may prove to be a synapomorphy for the entire tribe Xenurobryconini. In addition to providing information on reproductive morphology in these species, the results also demonstrate the potential use of characters obtained from such microscopic analyses in phylogenetic studies.