An acetyl fucoidan was isolated from Cladosiphon okamuranus which is commercially cultured, at 18,500 t/year off Okinawa Islands, Japan. The yield of acetyl fucoidan was 2.3% (W/W/) based on the wet algae. The total carbohydrates, D-glucuronic acid, sulphuric acid, ash, and moisture of the fucoidan was 69.0 %, 13.5 %, 13.6 %, 23.0 %, and 3.2 %, respectively. The L-fucose, D-xylose and D-glucuronic acid residues were identified by liquid chromatography. The infrared spectrum and optical rotation of the fucoidan was in agreement with that of standard fucoidan (Sigma Co., Ltd.). The acetyl residue was identified by 1 H-NMR and 13 C-NMR spectroscopy, respectively. The molar ratio of the sugar residues of the acetyl fucoidan was estimated to be L-fucose:D-xylose; D-glucuronic acid: acetic acid: sulfuric acid, 4.0:0.03:1.0:2.0:2.0. An alginate was also isolated from Cladosiphon okamuranus . The yield of the alginate was estimated as 0.1% (W/W) based on the wet algae. The total carbohydrate, uronic acid, ash, and moisture of the alginate was 87.6 %, 87.9 %, 10.0 %, and 2.3 %, respectively. The infrared spectrum and optical rotation of the alginate was in agreement with that of standard alginate. The d-mannuronic acid and L-guluronic acid were identified by 1 H- and 13 C-NMR spectroscopy, the molar ratio of both sugar residues was estimated to be 0.20:1.00. The acetyl fucoidan could be produced simply from artificially cultured Cladosiphon okamuranus .