Abstract: The pineal gland via its secretory product, melatonin, influences the light–dark rhythm in most vertebrates including fish. Apart from the information concerning this circadian rhythm, the interrelation of the melatonin with other physiological processes has not been considered in fish. Thus, we evaluated the changes in the humoral innate immune system of seabream (Sparus aurata L.) and sea bass (Dicentrarchus labrax L.) specimens exposed to a constant light–dark photoperiod (12 hr L:12 hr D). Serum was obtained from blood samples collected at 02:00, 08:00 hr (light-on), 14:00, 20:00 hr (light-off) and at 08:00 hr again. Among the humoral innate immune responses, complement, lysozyme and peroxidase activities were determined. Complement activity was higher during the day than during the night in both fish species. Seabream lysozyme activity reached its maximum at 20:00 and 02:00 hr but was hardly affected in sea bass. Finally, the peroxidase activity of seabream was significantly higher at 08:00 hr than during the rest of the cycle while, in sea bass, it showed little variation. The present results demonstrate that the humoral innate immune system has a circadian rhythm based on the light–dark cycle and that this cycle might be affected by the pineal gland.