Abstract Osmolality, the concentrations of glucose, total cholesterol, and triglyceride, and the concentrations of ions such as Na + , Cl − , K + , and Mg 2+ in the seminal plasma of yamú Brycon amazonicus were evaluated during one reproductive season. Semen quality and the relationship between spermatocrit and sperm concentration were examined over two reproductive seasons. Activation time, spermatocrit, and sperm concentration were significantly higher in 2003 than in 2004. Spermatocrit and sperm concentration showed a significant positive relationship ( r 2 = 0.79). During the 2004 reproductive season, a significant variation was observed for spermatocrit and sperm concentration between the initial period (February 15th to March 14th) and the middle and final periods (March 15th to April 14th and April 15th to May 14th, respectively). Neither the ion concentrations nor the glucose and cholesterol concentrations varied between any periods of the reproductive season. Osmolality and triglyceride concentrations showed significant variations between periods during the reproductive season.