Heavy mortalities were observed in rainbow trout (Oncorhynchus mykiss) reared in floating cages of five fish farms in the Black Sea, Turkey, in April 2004. Microbial, histological, hematological, and electron microscopy were used to determine the reason for these mortalities. The fish were affected with erythrocytic inclusion body syndrome (EIBS). One or two pathogenic bacteria were isolated from each sampled fish. The bacteria were identified as Aeromonas hydrophila, Flexibacter maritimus, Listonella (Vibrio) anguillarum, and Pseudomonas fluorescens. Erythrocytes of the affected fish had single, double, or multiple inclusion bodies. Necrotic and deformed red blood cells (RBC) had segmented or swollen lysed (karyolitic) nuclei. The virions had a diameter of 75-100 nm and appeared to be five or six-sided. Individual virions had a spherical appearance and were scattered throughout the cytoplasm. Diseased fish had low hematocrit values (Ht), low RBC counts, and usually high leukocyte counts. This is the first time that EIBS associated with pathogenic bacteria was evidenced in marine reared rainbow trout in Turkey.