The marine photosynthetic bacterium, Rhodovulum sp. PS88, produces RNA not only in cells but also as an extracellular polymeric substance during aerobic continuous cultivation in the dark. At a dilution rate of 0.32-0.5 h(-1), the maximum RNA production was 460 mg RNA l(-1) broth (200 mg RNA g(-1) suspended solids) which is a value about 2-3 times more than that of yeast cells.