Methanosarcina semesiae MD1T (T l type strain), a novel obligately methylotrophic methanogenic archaeon is described. Strain MD1 T was isolated from an enrichment on dimethylsulfide inoculated with mangrove sediment.The cells were irregularly coccoid, non-motile, 14O02 µm in diameter and stained Gram-positive. The catabolic substrates used included dimethylsulfide, methanethiol, methanol and methylated amines, but not acetate, formate, H 2 /CO 2 or a combination of these substrates. When cells grown on dimethylsulfide were transferred to trimethylamine or methanol and vice versa, a lag phase was observed. The same lag phase occurred when cells grown on trimethylamine were transferred to methanol and vice versa, indicating that for each substrate different enzymes were induced. Fastest growth occurred within a temperature range of 30-35 mC and a pH of 65-75. Both Na M and Mg 2M were required for growth, with maximum growth rates at 200-600 mM Na M and 20-100 mM Mg 2M . The cells exhibited specific growth rates (h N1 ) of 007O002, 015O004 and 018O005 on dimethylsulfide, methanol and trimethylamine, respectively. Analysis of the 16S rRNA gene sequence showed that strain MD1 T was phylogenetically closely related to members of the genus Methanosarcina, but clearly differed from all described species of this genus (94-97 % sequence similarity).