ABSTRACT: Enterococcus seriolicida is the causative agent of an enterococcal infection in yellowtail Seriola quinqueradiata and is antigenically classified into 2 phenotypes, KG+ and KG-. Phenotypic variation from KG- to KG+ occurs readily on an artificial medium. The surface morphologies of the KG- and KG+ phenotypes were differentiated by scanning electron microscopy. KG- cells were more hydrophilic than KG+ cells and were resistant to phagocytosis by yellowtail head kidney phagocytes. The chemiluminescent response of these phagocytic cells was lower with the KG- phenotype than with the KG+ phenotype. The immune responses of yellowtail following injection of the 2 phenotypes differed: higher agglutinating titers were obtained with the KG+ phenotype compared to the KG- phenotype. Thus, the cell surface may function as an anti-phagocytic factor in E. seriolicida and apparently affects its immunogenicity.