- The aim of this study was to evaluate the sperm quality of Brazilian flounder Paralichthys orbignyanus throughout its reproductive season. Sperm was collected at the beginning, middle and end of the breeding period. Spermatozoa density was maximum at the beginning (12.7 ± 0.92 x 10⁹ cells mL⁻¹) and at the end (11.8 ± 0.39 x 10⁹ cells mL⁻¹) of the breeding season (P<0.05). Sperm production and the percentage of spermatozoa moving fast forward increased significantly towards the end of the breeding season (P<0.05). The mean duration of progressive motility of spermatozoa was around 10 min. No difference was observed during the reproductive season in the percentage of motile cells, pH, osmolality and K⁺, Cl⁻ and Mg²⁺ concentrations in seminal plasma. The concentration of Na⁺ increased throughout the breeding season, reaching 174.62 ± 12.68 mmol L⁻¹ at the end (P<0.05). There was a decline in the concentration of Ca²⁺ (12.31 ± 3.08 mmol L⁻¹) in the middle of the breeding season, which coincided with the shortest motility duration of spermatozoa. The information reported in this study should help to improve management and optimize the development of protocols for short-term storage and cryopreservation of Brazilian flounder semen.