The first observed pasteurellosis outbreak at a low marine water temperature (18-19 degrees C) is hereby reported. The disease was identified in three sea bass farms in the Aegean near Bodrum, Turkey. Diseased fish, Dicentrarchus labrax (L.), were characterized by lethargy, loss of appetite, darkened skin color, exophthalmia, abdominal swelling, pale gills, and hemorrhages over the operculum and ventral part of the body. Some diseased fish had hemorrhages on the head and opaque eyes. There were whitish nodules in the liver, the spleen varied in size (0.5-4 mm), and both organs were pale. Morphological, physiological, and conventional biochemical tests were used to determine the phenotypic properties of pure cultures of isolated colonies in samples taken from internal organs. Mono-Pp agglutination kit and API 20E were used to confirm the identified bacteria. The isolated strain was Photobacterium damselae subsp. piscicida. The sensitivity of the Aquarapid-Pp kit, which produced positive results in all diseased fish, was 0.91 and the specificity was 0.95. The principal histological changes were depletion of hemopoietic tissue and multiple round small or large necrotic and lytic areas in the spleen, vacuole degeneration and diffuse hemorrhagie in the liver, and peritubular vacuole degeneration and tubular necrosis in the kidney. Treatment with flumequine in the feed at 50 mg/kg body weight/day for seven days controlled mortality.