Abstract: The humoral immune responses and host protection of pompanos ( Trachinotus ovatus),against Cryptocaryon irritanswere determined after immunization with live theronts and killed theronts of. C. irritans. Two groups of pompanos (150 animalsper group) were immunized three times with live theronts of C. irritans by immersion at a dose of 8 103 theronts per fish or withkilled theronts of C. irritans by intraperitoneal (i. p. ) injection at a dose of 115 105 theronts per fish. The immobilization titersof antiserum and skin culture supernatant of the immunized fish were determined with an immobilization assay every two weeks.Also the immunized fish were challenged with a sublethal dose of theronts (6 103 theronts per fish) or lethal dose of theronts(2 104 theronts per fish) in Week 14. The level of C. irritans infection (indicated by relative infection level,RII) and survivalof pompanos were determined after C. irritans challenge. The results showed that the specific immobilizing antibodies could bedetected in both antiserum and skin culture supernatant in the immunized fish.Both immunization methods (immersion with livetheronts or i. p. injection with killed theronts) could confer protection to fish because when all fish were challenged with a lethaldose of theronts of C. irritans, some of the immunised fish survived,but all control fish died. There was a positive correlation be2tween higher levels of the specific immobilizing antibodies and host survival rate in the immunized fish. However,compared withi. p. injection with killed theronts,immersion with live theronts was more effective in stimulating immune responses,which indicat2ed by the higher immobilization titers in both antiserum and skin culture supernatant and the higher survival rate of the immunisedfish.When fish immunised with live theronts and killed theronts were challenged with a sublethal dose of theronts of C. irritans,the reduction rate of RII of C. irritans were 5016 % and 3712 % respectively. When fish were challenged with a lethal dose oftheronts of C. irritans,the survival rate of fish immunised with live theronts and killed theronts were 60 % and 40 % respectively.In both groups of fish,antibody levels in skin culture supernatant were much lower than those in serum. The immobilizing antibod2ies in serum and the immobilizing antibodies in skin culture supernatant of the immunized fish were different in terms of onset ofimmunity,timing,pattern of the peaks,and immobilization titers. Therefore,it is speculated that pathway of the systemic immuneresponses and mucosal immune responses were different. The present study indicates that humoral immune response,especiallymucosal immune response,plays an important role in protection against C. irritans infection.Vaccination may be an effective wayto prevent white spot disease of marine fish.