Abstract A marine bacterium degrading the water-soluble cell wall polysaccharides from Ulva sp. (ulvan) has been isolated. The good correlation between ulvan degradation monitored by reducing-power, UV absorbance and viscosimetry, indicated that the crude enzymatic extract contains essentially an endo-ulvan lyase activity. This activity was rapidly inhibited by the reaction products which consisted of a series of ulvanobiouronic acid A 3- sulfate [→ 4)-β- d- Glcp A-(1 → 4)-α- l- Rhap 3- sulfate-(1 → ]n with 4-deoxy- l- threo-hex-4-enopyranosiduronic acid at the non-reducing end. Other deviant repeating structures with β- d-Xyl p or α- l-Ido p A replacing β- d-Glc p A in the repeating ulvanobiouronic acid disaccharide and the presence of two consecutive (1 → 4) linked β- d-Glc p A demonstrated the great variability and complexity of ulvan chemical structure.