Mangrove forest sediments produce significant amounts of methane, but the diversity of methanogenic archaea is not well known at present. Therefore, 16S rRNA gene libraries were made using archaea-specific primers and DNA extracted directly from Tanzanian mangrove sediment samples as a template. Analysis of sequence data showed phylotypes closely related to cultivated methylotrophic methanogenic archaea from the marine environment, or distantly related to acetoclastic and hydrogenotrophic methanogenic archaea. In an attempt to isolate relevant methanogenic archaea, we succeeded in obtaining a new mesophilic methylotrophic methanogenic archaeon (strain MM1) capable of utilizing methanol and methylated amines as the only substrates. Under optimum conditions, the cells of strain MM1 exhibited a high specific growth rate (mu) of 0.21+/-0.03 (i.e. doubling time of 3.2 h) on both methanol and trimethylamine. The 16S rRNA gene sequence of strain MM1 clustered with five environmental clones, indicating that MM1 is an important methanogenic methylotroph in mangrove sediments. Based on physiological and phylogenetic analyses, strain MM1 is proposed to be included in the species of Methanococcoides methylutens.