Previously, a psychrophilic rod-shaped marine bacterium (strain HAW-EB3T) isolated from Halifax Harbour sediment was noted for its ability to degrade hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). In the present study phenotypic, chemotaxonomic and genotypic characterization showed that strain HAW-EB3T represents a novel species of Shewanella. Strain HAW-EB3T contained lysine decarboxylase, which is absent in other known Shewanella species, and distinguished itself from most other species of Shewanella by the presence of arginine dehydrolase, ornithine decarboxylase and chitinase, and by its ability to oxidize and ferment N-acetyl-D-glucosamine. Strain HAW-EB3T grew on several carbon sources (N-acetyl-D-glucosamine, Tween 40, Tween 80, acetate, succinate, butyrate and serine) and showed distinctive fatty acid and quinone compositions. Both phenotypic and 16S rRNA gene phylogenetic cluster analyses demonstrated that HAW-EB3T belongs to the Na+-requiring group of Shewanella species. The HAW-EB3T 16S rRNA gene sequence displayed <=97·4 % similarity to all known Shewanella species and was most similar to those of two bioluminescent species, Shewanella hanedai and Shewanella woodyi. However, gyrB of strain HAW-EB3T was significantly different from those of other Shewanella species, with similarities less than 85 %. DNA-DNA hybridization showed that its genomic DNA was less than 25 % related to that of S. hanedai or S. woodyi. Therefore we propose Shewanella sediminis sp. nov., with HAW-EB3T (=NCIMB 14036T=DSM 17055T) as the type strain.