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In this study, we explored variation in sperm morphometry of the African cyprinid Barbus neumayeri Fischer, 1884 (Neumayer's barb) across seven sites with a wide range in dissolved oxygen, from hypoxic swamps to intermittent normoxic streams to well-oxygenated rivers. We explore whether fish physiological condition ( K) or hypoxia can affect the reproductive traits, and whether condition-hypoxia dependence of sperm traits including head length ( LH), head width ( WH), flagellum length ( LF), and hydrodynamic ratio (HR) vary across sampling sites. Significant differences were found in fish total length ( P = 0.0212), as well as in K, left and right testis masses, total gonad mass, and gonadosomatic index ( P < 0.0001 for all traits). Total gonad mass was lower in hypoxic sites than in well-oxygenated sites. Interestingly, the left and right testes from normoxic environments were double the size of testes from hypoxic environments. Despite little variation in sperm flagellum length, sperm heads were longer in swamps than in streams or rivers, giving the sperm head a more hydrodynamic shape. This variation in HR may be beneficial in the more stagnant waters of the swamp compared with the other environments. Future studies are necessary to understand whether variation in sperm morphology correlates with sperm swimming performance and male reproductive capacity. [ABSTRACT FROM AUTHOR]