Two marine bacterial strains, KMM 3582T and KMM 3589, isolated respectively from sediments of the South China Sea and sea water of the Sea of Japan, have been characterized. Comparative 16S rDNA sequence-based phylogenetic analysis placed the two strains in a separate branch of the gamma-Proteobacteria within the members of the genus Shewanella. KMM 3582T showed the highest similarity (97.1 and 97.4%, respectively) to Shewanella pealeana and Shewanella gelidimarina. The G + C contents of the DNAs of the two strains studied were 45.0 mol%. The level of DNA-DNA relatedness between the two strains was 82%, indicating that they represent a single genospecies. These organisms were slightly pinkish, Gram-negative, polarly flagellated, facultatively anaerobic, mesophilic (with temperature range from 4 to 30 degrees C), neutrophilic and haemolytic and were able to degrade alginate, gelatin and DNA. The novel organisms were susceptible to gentamicin, lincomycin, oleandomycin, streptomycin and polymyxin. The predominant fatty acids were characteristic for shewanellae: 13 : 0-i, 15 : 0-i, 16 : 0 and 16 : 1omega7. Eicosapentaenoic acid, 20 : 5omega3, was not detected. Phylogenetic evidence, together with phenotypic characteristics, showed that the two bacteria constitute a novel species of the genus Shewanella. The name Shewanella fidelis sp. nov. is proposed, with the type strain KMM 3582T (=LMG 20551T =ATCC BAA-318T).