Biomethane produced from seaweed may be used as a transport biofuel. Seasonal variation will have an effect on this industry. Laminaria digitata, a typical Irish brown seaweed species, shows significant seasonal variation both in proximate, ultimate and biochemical composition. The characteristics in August were optimal with the lowest level of ash (20% of volatile solids), a C:N ratio of 32 and the highest specific methane yield measured at 327 L CH4 kg VS-1, which was 72% of theoretical yield. The highest yield per mass collected of 53 m(3) CH4 t(-1) was achieved in August, which is 4.5 times higher than the lowest value, obtained in December. A seaweed cultivation area of 11,800 ha would be required to satisfy the 2020 target for advanced biofuels in Ireland, of 1.25% renewable energy supply in transport (RES-T) based on the optimal gross energy yield obtained in August (200 GJ ha(-1) yr(-1)). (C) 2016 Elsevier Ltd. All rights reserved.