Abstract.Trials were run to determine the immunomodulatory effect of dietary vitamin C in rainbow trout.Oncorhynchus mykiss(Walbaum), and Atlanticsalmon,Salmo SalarL. We first investigated whether depletion of ascorbic acid (AA) could depress the immune system. Subsequently, we focused our studies on the stimulatory effect of a high dietary dose of stable vitamin C derivatives (1000mg AA‐equivalent per kg of feed) in order to determine their prophylactic role.Three trials were run with rainbow trout fed, for periods of time varying from 2 to 6 months, a high dietary dose of ascorbate‐2‐polyphosphate (AAPP) or L‐ascorbate‐2‐monophosphate calcium salt (AAMP) equivalent to 1000mg AA per kg of feed. The high dietary level of vitamin C was compared to 60mg AA‐equivalent per kg of feed. Another experiment was conducted with Atlantic salmon pre‐fed AAPP at 60mg/kg for 6 months followed by AAPP at 1000mg/kg for 2 months. Various immune parameters were tested such as mitogen‐induced proliferation, serum complement level, phagocytosis and natural cytotoxicity. Leucocyte and plasma AA contents were also determined.The immune response in vitamin‐C deficient trout was not strongly different from that in trout fed the minimal growth requirement level. Serum complement level was not affected by vitamin C. A high dietary dose of vitamin C was found to increase trout and salmon leucocyte AA content, mitogen‐induced proliferation of lymphocytes, and chemiluminescence response of leucocytes with a higher constancy, when the cells were stimulated with opsonized zymosan and with luminol as a substrate. Natural cytotoxicity was enhanced in trout fed a high dietary dose of vitamin C. The results of these immunity studies support supplementation levels of vitamin C which are much higher than the minimal requirement known to prevent deficiency symptoms and promote normal growth. Further investigations on the time‐dose response are needed to define quantitatively the high di