This paper describes the first occurrence of viral erythrocytic infection associated with pathogenic bacteria and some ectoparasites in cultured Mediterranean sea bass (Dicentrarchus labrax) in Turkey. Generally, fish infected by the organisms were anaemic, and experienced moderate or high levels of mortality during disease outbreaks. While Photobacterium damselae subsp damselae, Aeromonas salmonicida subsp. salmonicida, Aeromonas sobria, Listonella (Vibrio) anguillarum, and Staphylococcus epidermidis were isolated from diseased fish, haematological and electron microscopical examination also indicated the presence of viral erythrocytic infection in diseased sea bass. Erythrocytes of affected sea bass had. single, double or multiple intracytoplasmic inclusions ranging in size from 0.3 to 1.6 mu m. Some of these erythrocytes had single or multiple, small or large vacuoles which were usually associated with inclusion bodies. Confirmation of viral erythrocytic infection is based on the observation of polyhedral virions 85-157 nm in diameter in the cytoplasm of the affected erythrocytes using electron microscopy. Clinically infected sea bass had low haematocrit values (Ht) ranging from 2% to 29% and low RBC counts ranging from 14,200 to 3,760,000 per ml.