Aphanocapsa halophytia MN-11 isolated from a hypersaline environment was found to produce large quantities of exopolysaccharide. We-describe here production of exopolysaccharide and the characterization of its properties. The effects of medium composition, particularly NaCl concentration, were tested, Maximum exopolysaccharide production was obtained with nitrogen and phosphorus concentrations in the medium exceeding 100 and 40 mg.L(-1) respectively. In addition, when 30 g.L(-1) NaCl was added to the medium, exopolysaccharide could be recovered from the medium supernatant. Exopolysaccharide from this strain was made up of at least six mono-oses and did not contain uronic derivatives or osamines. Proteins represented about 10% of total weight and, interestingly, 12% (wt/wt) sulfated residues, which is unusual for photosynthetic prokaryotes.