Seaweed may be a source of third generation gaseous biofuel, in the form of biomethane. The scientific literature is sparse on the relative suitability of different varieties of seaweed to produce biomethane. This paper assesses the BMP (biochemical methane potential), ultimate analysis and theoretical yields of ten species of seaweed which may be found in commercial quantities around the coastline of Ireland. Saccharina latissima reported the highest BMP yield (ca. 342 L CH4 kg VS-1). S. latissima if farmed, may produce 10,250 m(3) CH4 ha(-1) yr(-1) (365 GJ ha(-1) yr(-1)) which is in excess of all land based liquid biofuel systems. (C) 2014 Elsevier Ltd. All rights reserved.