ABSTRACT: Mortality among hatchery-reared juvenile white seabass Atractoscion nobilis in southern California, USA, was associated with infections by a Piscirickettsia salmonis-like organism (WSPSLO). Infected fish had no consistent external signs other than pale gills, lethargy and impaired swimming behavior. Internally, the kidney and spleen were enlarged, and some fish had livers with multiple pale foci. Smears from infected kidney, liver, and spleen stained with Wright-Giemsa had intracytoplasmic coccoid organisms, often in pairs, that ranged in size from 0.5 to 1.0 µm. Microscopic lesions included multifocal hepatic, renal, and splenic necrosis, and intralesional macrophages often contained the WSPSLO. The bacterium was isolated from infected fish on cell lines of salmonid (CHSE-214) and white seabass (WSBK) origin. The WSPSLO induced plaque formation and destroyed the cell monolayers within 10 to 14 d incubation at temperatures of 15 and 20°C. The bacterium retained infectivity for cell lines up to 14 d at 4 and 13°C, up to 7 d at 20°C, but it was inactivated at 37 and 56°C within 24 and 1 h, respectively. Freezing at -20°C reduced infectivity by 100-fold. Dehydration and resuspension in distilled water completely inactivated the bacterium. In contrast, the WSPSLO retained nearly all of its infectivity for CHSE-214 cells following a 72 h period in seawater at 20°C. Polyclonal rabbit antibodies made to the WSPSLO reacted specifically in indirect fluorescent antibody tests (IFAT) with the bacterium in cell cultures and smears from infected fish tissues. Tissue smears from infected salmon or CHSE-214 cells with P. salmonis reacted weakly with the anti-WSPSLO serum. Conversely, polyclonal anti-P. salmonis serum produced a weakly positive reaction with the WSPSLO from infected CHSE-214 cells. The WSPSLO as propagated in CHSE-214 cells was highly virulent for juvenile coho salmon Oncorhynchus kisutch, inducing 80% mortality within 10 d of intraperitoneal injection of 10^2.5 50% tissue culture infectious doses per fish. We conclude that the bacterium from white seabass possesses antigenic differences from P. salmonis yet possesses virulence for salmon equal to known strains of P. salmonis.