Gram Positive Streptococci (GPS) have become increasingly important fish pathogens. In this study, 170 moribund marine fish (sea bass, gilthead sea bream, meagre, sharp snout sea bream, and marine rainbow trout), obtained from 30 fish farms in the Black Sea and Aegean Sea regions of Turkey, were investigated for the presence of GPS. Streptococci were recovered from fifteen moribund young meagre (Argyrosomus regius) from an inland fish farm located in the Aegean region. The isolates were identified as Enterococcus casseliflavus using a Polymerase Chain Reaction-based (PCR) method. This result was confirmed with 16S rRNA gene sequencing and serological methods, including slide agglutination and IFAT. The diseased meagre showed unilateral or bilateral exophthalmia, loss of eyes, and hemorrhagic ulcerative skin lesions on the body surface. Histopathologically, tubular degeneration and congestion in the kidney, myopathy in the heart tissue, discharge of the white pulp in the spleen, extensive vacuolation of hepatocytes, and congestion in the liver were observed. Intraperitoneal challenge with one of the isolates in healthy European sea bass (Dicentrarchus labrax) in resulted mortalities 8 days post-injection.