Attempts were made for the isolation of organisms that caused red staining of culture bed (cremona strings) of sporophytes of makonbu and falling of sporophytes from the red-spotted culture bed. Nine strains of Alteromonas sp. were isolated from red-spotted culture beds in the three different nurseries, and were shown to produce prodigiosin-like pigment on double layer agar plates as well as cremona strings covered by soft agar layer, containing viable cells of Escherichia coli. The isolates failed to utilize cell-components of makonbu, such as mannitol, alginate, laminarin and cellulose, but were able to lyse the viable cells of other gram-negative bacteria. According to these properties, it appeared that the isolates grow at the expense of other viable gram-negative bacteria on the cultured bed by lysing them, and produce prodigiosin-like pigment resulting in the staining of cultured bed.