Spot-disease of Laminaria fronds has been occasionally observed in coastal areas of Hokkaido Japan. A huge outbreak of the disease occurred in Laminaria japonica near the southeastern part of Hokkaido in 1998. Three bacterial strains showing Laminaria-frond degrading activity were isolated from the spot-wounded fronds of L. japonica collected at Minamikayabe, Hokkaido, Japan. All the strains were tentatively identified as Alteromonas strain, and two of them were identified as Pseudoalteromonas elyakovii, of which the species included an alginolytic marine bacterium isolated from a spot-disease of Laminaria japonica var. ochotensis occurring at Rishiri Island at 1985. The phenotypic and genotypic characterizations of the Laminaria-frond degrading bacteria were described.