Abstract: To investigate the effects of a Cryptocaryon irritans infection on the biochemical functions and immune responses of the marbled rockfish Sebastiscus marmoratus, this study utilized C. at concentrations of 2,500, 5,000, 7,500, and 10,000 theronts/fish to infect marbled rockfish weighing (453) g. Within 96 hours after the infection we measured a variety of parameters at different time points, including (in the serum) cortisol (COR), glucose (GLU), total protein (TP) contens, (in the liver) the malondialdehyde (MDA) and vitamin C (VC) contents, the superoxide dismutase (SOD) and catalase (CAT) activities, and (in the gill and skin) the lysozyme (LZM) activities of the marbled rockfish. The results showed that the serum COR and GLU contents of the marbled rockfish gradually increased along with the infection concentration and the lapse of time after the infection. However, the TP content decreased while infection concentration increased. Compared to the control, MDA contents in the liver decreased significantly in the 5 000 theronts/fish infection group, then increased significantly and reached the maximum in the 10000 theronts/fish infection group. VC contents in the liver initially rose and then dropped. Compared to the control group, the SOD and CAT activities of the fish were significantly elevated. The LZM activities in the gill and skin initially increased and then decreased. In conclusion, C. irritans infection may result in adverse effects on the rockfish, probably through oxidative stress and lipid peroxidation. The physiological functions of the fish infected with low concentrations of C. irritans theronts could be effectively restored after the removal of trophont from the host; however the infection with high concentrations irreversibly impaired the release of immune factors.