This study compared single- and two-stage mesophilic fermentation of Laminaria digitata. The two-stage system comprised a hydrolysis reactor (H-1) with a hydraulic retention time (HRT) of 4 days followed by two methanogenesis reactors M-1 and M-2 with HRT of 20 and 14 days respectively. The single-stage reactor (M-3) had an HRT of 24 days. Specific methane yields of 176, 234 and 221 Likg VS were obtained for Mi, M-2 and M-3. The methane concentration of the biogas was 22% higher for the two-stage system (58-61%) than the single-stage system (50%). Hydrolysis yielded a hydrogen yield of 26 L/kg VS. The two-stage system (H-1, M-2) provided an overall energy yield of 8.66 MJ/kg at an HRT of 18 days compared to 7.89 MJ/kg in the single-stage system with an HRT of 24 days. Thus, the two-stage system reduced the HRT by 33% whilst improving the energy conversion by 9.8%. (C) 2017 Elsevier Ltd. All rights reserved.