Abstract Zostera marina was investigated for ethanol production. The examined plant contained 30% of glucan. It was pretreated by steam explosion and the best conditions were assessed by a 23 full factorial design with repeated centre point. Temperature, time of pretreatment and oxalic acid load were selected as experimental factors. The experimental data of water-soluble sugar recovery and enzymatic saccharification of the insoluble residue were analysed by the response surface regression procedure, which provided significant models with R2=0.98. The best results (5.06 g of soluble sugars and 52.9 g of glucose, respectively, from 100 g of exploded material and 100 g of insoluble fibre) were attained at pretreatment of 180 °C, 300 s and 2 wt% of oxalic acid. Fermentation tests of the hydrolysed fibre were carried out and the ethanol production was optimised by varying the enzyme load, the amount of yeast and the solid concentration. In the best cases 243 g of ethanol was produced per kg of Zostera fibre and the concentration of 4.7 v% was attained in tests of high-solid load.