An aerobic, polarly flagellated marine bacterium that produces a prodigiosinlike pigment was isolated from the red-spotted culture beds of Laminaria japonica. Five isolates had unique bacteriolytic activity for both Gram-positive and -negative bacteria, which had never been observed among Alteromonas or related species. The isolates were identified as the causative agent of red spot disease of L. japonica seeds. The phenotypic features of the isolates were similar to these of Pseudoalteromonas rubra ATCC 29570T, but they could be differentiated using 10 traits (growth at 37 "C, requirement for organic growth factors, bacteriolytic activity, utilization of sucrose, N-acetylglucosamine, fumarate, succinate, D-galactose, L-proline and acetate). The G+C content of DNAs from the isolates was 4 4 4 6 mol0/o. The isolates constitute a new species, distinct from the other A/teromonas and Pseudoalteromonas species, as shown by DNA-DNA hybridization experiments and phylogenetic clustering of 165 rRNA gene sequences, for which the name Pseudoalteromonas bacteriolytica sp. nov. (type strain = IAM 145953 is proposed. A set o f phenotypic features which differentiate this new species from closely related Pseudoalteromonas and Akeromonas species is provided.