Two bacterial strains isolated from the Baltic Sea, OS145T and OS146, were characterized on the basis of their physiological and biochemical features, their fatty acid profiles and their phylogenetic position based on 16S rDNA sequence analyses. The strains were isolated from the upperoxic water column of the central Baltic Sea. Phylogenetic analyses of the 16S rDNA gene sequences revealed a clear affiliation of the novel strains with members of the genus Idiomarina, of the Gammaproteobacteria. Closest sequence similarity was seen with Idiomarina abyssalis and Idiomarina zobellii (95-96%). The mean G + C content of the DNA of strains OS145T and OS146 was 49.7 mol%. Both strains were non-pigmented, Gram-negative, polarly flagellated organisms that were strictly aerobic. Growth of the strains was observed at salinities ranging from 0.8 to 10% NaCl. Temperature range for growth was rather broad and high for marine bacteria: both strains grew between 8 and 46 degrees C, showed good growth between 20 and 44 degrees C, and had an optimum between 30 and 40 degrees C. The fatty acids of the two strains were dominated by iso-branched fatty acids (54-80%), with a high abundance of C15:0 iso (36%), C16:1 omega7c, C17:o iso and C17:1 iso omega9c. Growth temperature (8-40 degrees C) influenced the fatty acid composition of the strains in a way that the content of iso-branching fatty acids increased with increasing temperatures, while the mono-unsaturated fattyacids increased with decreasing temperatures. Salinity (1.7-10% NaCl) had only a minor effect on the fatty acid composition. According to their morphology, physiology, fatty acid composition and 16S rDNA sequences, strains OS145T and OS146 fitted well into the genus Idiomarina, but could be easily distinguished from the recognized species of the genus. Because of their unique nature, it is proposed that the strains isolated from the Baltic Sea represent a novel species, for which the name Idiomarina baltica (type strain OS145T =DSM 15154T =LMG 21691T) is proposed.