On the basis of phenotypic and genotypic characteristics and analysis of 16S rRNA sequences, a novel species belonging to the genus Pseudoalteromonas is described. Two pale-orange-pigmented strains, KMM 300(T) and KMM 290, isolated respectively from a mussel, Crenomytilus grayanus, and a scallop, Patinopecten yessoensis, are marine, Gram-negative, aerobic, rod-shaped bacteria that produce a number of antimicrobial compounds. The strains are able to degrade gelatin, elastin, starch, DNA and Tween 80. Chitin and agar are not degraded. The isolates from marine invertebrates grew at NaCl concentrations of 1--9% and a temperature range of 10--35 degrees C and did not utilize most of the wide range of carbohydrates tested, with the exception of D-glucose, cellobiose and sucrose. The DNA G+C content was 48.4--48.9 mol%. The level of DNA homology of the two strains was 98%. DNA from the strains isolated from marine invertebrates showed 5--15% genetic relatedness to the DNA of other type strains of the genus Pseudoalteromonas. 16S rRNA analysis indicated a clear affiliation of the novel bacteria to other species of the genus. The strains are assigned to a novel species, Pseudomonas ruthenica sp. nov., with the type strain KMM 300(T) (=LMG 19699(T)=CIP 106857(T)).