Degradation of alginate and its constituents, polymannuronate (polyM) and polyguluronate (polyG), by gut bacteria isolated from sea urchins and abalones in the northern part of Japan, were investigated. Bacterial counts in the guts of sea urchin S. intermedius, were 10(5) to 10(8) CFU/g, and in abalone H. discus hannai, counts ranged from 10(6) to 10(9) CFU/g. More than 80% of total 600 isolates were found to have alginolytic activity. The alginolytic bacteria were predominantly fermentative, but some differences were observed in their substrate specificity as well as between the flora in the gut of sea urchins and the abalones. Seventy percent of the alginolytic bacteria from the sea urchins showed no degrading preference for polyM or polyG blocks, and were able to degrade both the substrates simultaneously. Most of the alginolytic bacteria (96.6%) from sea urchins belonged to the genus Vibrio. The majority of alginolytic bacteria (68.0% on average) from abalones only degraded polyG and they were predominantly non-motile fermenters. From these results, it appeared that a different type of association exists between alginolytic gut microflora and the marine algal feeders with respect to the level of contribution by bacteria to the host's digestion of alginate.