AbstractEnterococcus seriolicidastrains were divided into two groups, agglutinating and nonagglutinating, by a slide agglutination test using antiserum against the YT‐3 strain. Intraperitoneal injection of agglutinating and nonagglutinating strains into yellowtail,Seriola quinqueradiataTemminck&Schlegel, revealed that nonagglutinating strains were more virulent than agglutinating strains. Two nonagglutinating and highly pathogenic strains SS91–014 and SS91–092 were subcultured 30 times in brain heart infusion broth, and the agglutination titres of 50 colonies from subcultures 1, 2, 3, 4, 5, 6, 11, 16, 21, 26 and 30 against anti‐YT‐3 serum were determined. Transformation from a nonagglutinating (1:<4) to an agglutinating (1: ≥4) pattern was first observed at the sixteenth subculture, and the ratio of agglutinating to nonagglutinating substrains rose until the thirtieth subculture. At this time, 70% of the SS91–014 population and 52% of the SS91–092 population were transformed to an agglutinating pattern. When the pathogenicity of four transformed substrains with different agglutination titres was tested in yellowtail, the nonagglutinating substrain showed higher pathogenicity than the agglutinating substrains, but no relationship between LD50values and the agglutination titres of transformed substrains was observed. The pathogenicity ofE. seriolicidaappears to be related to the agglutination pattern, although it was not demonstrated that this property is solely responsible fo