Enterococcus s e n o h c~d a is the causative agent of an enterococcal Infection in yellowtail Seriola q~llnqueradlata and IS antigen~cally classified into 2 phenotypes KG+ and KG-Phenotypic variation from K G to KG+ occurs readily on an artihcial medium The surface morphoIogies of the K G and KG+ phenotypes were differentiated by scanning electron microscopy KG-cells were more hydroph~lic than KG+ cells and were resistant to phagocytos~s by yellowtail head kidney phagocytes The c h e m~l u m~n e s c e n t response of these phagocytic cells was lower with the KG-phenotype than with the KG+ phenotype The immune responses of yello.ruta11 f o l l o~~i n g injection of the 2 phenotypes differed h~g h e r agglutinating titers were o b t a~n e d with the KG+ phenotype compared to the KG-phenotype Thus, the cell surface may function as an anti-phagocytic factor in E seriolic~da and apparently affects ~t s immunogeniclty KEY WORDS. Enterococcus senol~cida . Seriola quinqueradlata . Phagocytosis . Chemiluminescence . lmmune response

The protective effect of Vibrio anguillarum immunization administered by the direct immersion technique in ayu, Plecoglossus altivelis Temminck and Schlegel, was investigated. The activities of phagocytic cells were suppressed at the time of vaccination by previous bath exposures to a eyclophosphamide solution containing a concentration of 500mg/kg/ml on days 2 and 3 before vaccination. An artificial challenge was carried out 3 weeks after vaccination by bath exposure. The survival rate of the vaceine group pretreated with eyclophosphamide was 96%. In the group given traditional vaeeinc only, 73% survived; and in the non-vaccine control group, 10% survived. These results suggest that protective immunity of ayu vaccinated by the direct immersion technique may not implicate the early activities of phagocytic eells.