Cyanothece sp. 113, a unicellular, aerobic, diazotrophic and photosynthetic marine cyanobacterium, produced 22.34 g/l of exopolysaccharide in 11 days at 29 degrees C, aeration rate of 7.0 l/min and continuous illumination with 4300 lux. After purification, the spectra of UV, IR, H-1 NMR, C-13 NMR and GC-MS analysis showed that the purified exopolysaccharide was alpha-D-1,6-homoglucan. This is first report describing linear alpha-D-1,6-homoglucan exopolysaccharide produced by marine cyanobacteria. (c) 2006 Elsevier Ltd. All rights reserved.