A newly described strain of the genus Methanosarcina was isolated from submarine canyon sediments and is shown by comparative sequence analyses of 16S ribosomal DNA and the gene encoding methyl coenzyme M reductase, mcrI, to be a strain of Methanosarcina siciliae. Morphological and physiological characteristics are described. In contrast to the two previously described strains that grow exclusively on methanol, methylamines, and dimethylsulfide, M. siciliae C2J is also capable of growth on and methanogenesis from acetate. We propose that the species description for M. siciliae be amended to include aceticlastic strains.