High concentrations of ascorbic acid occur in scurvy-prone, teleost fish plasma. We quantified seasonal relationships between 1) dietary level of vitamin C and level of seminal plasma ascorbic acid, and 2) seminal plasma ascorbic acid concentration and sperm quality, in rainbow trout (Oncorhynchus mykiss). We maintained six groups of 2-yr-old rainbow trout on diets supplemented with 0, 30, 110, 220, 440, and 870 ppm ascorbyl monophosphate beginning in May 1992. Sperm were produced during the end of October 1992 through April 1993; we collected milt 13 times from 3-8 fish per treatment. We quantified ascorbic acid concentration in seminal plasma, and sperm concentration, motility, and weight. Seminal plasma ascorbic acid concentrations were affected directly by the ascorbyl monophosphate level in the diet. Seminal plasma ascorbic acid concentrations also were affected by season. Ascorbic acid deficiency did not influence semen quality (sperm concentration and motility) at the beginning of the spawning season. However, sperm concentration and motility in a group fed an ascorbic acid-free diet declined during the period of study. Ascorbic acid deficiency reduced both sperm concentration and motility, and thus fertility, of rainbow trout. These results indicate that vitamin C is important for male fish reproduction; the dietary requirement for seminal plasma ascorbic acid saturation exceeds that for optimum growth.