The present study assesses the in vivo effect of vitamin D(3) or cholecalciferol on some innate immune parameters of the gilthead seabream (Sparus aurata L.). Cholecalciferol was orally administered to seabream specimens in a commercial pellet food supplemented with 0 (control); 3750; 18,750 or 37,500 U kg(-1) and fish were sampled after 1, 2 and 4 weeks of treatment. Serum and head- kidney leucocytes were obtained and humoral (peroxidase and complement activity) and cellular (leucocyte peroxidase content, phagocytic, respiratory burst and natural cytotoxic activities) innate immune parameters were measured. Diet supplementation with 37,500 U kg(-1) cholecalciferol for 2 or 4 weeks resulted in a significant increase in phagocytic ability or serum peroxidase content, respectively, whereas the 3750 and 18,750 U kg(-1) supplemented diets led to significant increases in the phagocytic capacity of leucocytes at week 2 compared with the values found in control fish. Natural cytotoxic activity was increased in leucocytes from fish fed for 1 week with 3750 U kg(-1) cholecalciferol. No significant differences were observed in complement activity or in respiratory burst activity in the assayed conditions. These results suggested that dietary vitamin D(3) administration has an effect on the innate immune parameters of gilthead seabream. The immunostimulant effect was greater on the cellular innate immune parameters assayed, suggesting that similar receptors to those present in mammals are involved in the action of this vitamin in the fish immune system.