The phenotypic, genomic and phylogenetic characteristics of four aerobic, Gram-negative, non-fermentative, motile, non-pigmented, agarolytic Pseudoalteromonas-like bacteria, isolated from marine environments, have been investigated. These bacteria share DNA-DNA similarities above 86%. Comparative 16S rDNA sequence analysis of strain KMM 255T revealed its membership of the genus Pseudoalteromonas; it shares 99.9% sequence similarity with Pseudoalteromonas distincta, Pseudoalteromonas elyakovii, Pseudoalteromonas atlantica and Pseudoalteromonas espejiana. DNA-DNA reassociation levels obtained for strain KMM 255T and type strains of these four species and other Pseudoalteromonas species were below 45%. The marine isolates differed from known species of the genus by the fact that the cells are motile by means of a single flagellum or two to four polar unsheathed flagella and by an inability to utilize most organic compounds. On the basis of phenotypic, DNA-DNA hybridization and phylogenetic data, it is concluded that the isolates represent a novel species within the genus Pseudoalteromonas, for which the name Pseudoalteromonas agarivorans sp. nov. is proposed. The type strain is strain KMM 255T (= DSM 14585T).