Six cyanobacterial isolates recovered from Polynesian microbial mats, called "kopara," were cultured using laboratory-closed photobioreactors and were shown to produce exopolymers as released and capsular exopolysaccharides (EPS). These polymers have been chemically characterized using colorimetric and elemental assays, infrared spectrometry, and gas chromatography. Both capsular and released EPS consisted of 7 to 10 different monosaccharides with neutral sugars predominating. Interestingly, four isolates exhibited sulfate contents ranging from 6% to 19%. On the basis of preliminary data, cyanobacteria from this unusual ecosystem appear to be an important source of novel EPS of a great interest in terms of their biological activities.