The effect of the extracellular polymeric substance (EPS) derived from a marine photosynthetic bacterium, Rhodovulum sp., on the flocculation of kaolin is described. The amount of EPS obtained and its flocculating activity were highly dependent on the extraction temperature and duration: high flocculating activity was obtained at a relatively low temperature (30 degrees C) and long extraction time (60 min). The EPS was classified as a weak-anionic high-polymer group based on its pH profile, and its promoting effect on flocculation was dependent on the concentration of cationic ions and their valence. The relative influence of cations on the critical flocculating concentration (Cs, cr) followed the Schulze-Hardy rule for colloidal flocculation. This suggests that the promotion of flocculation by the additions of EPS and cations occurred as a result of the synergistic effect of the release of electrical repulsion by the cations and the formation of bridges between the EPS and the cations.