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Author(s): Rafael Laso-Prez [1, 2]; Gunter Wegener (corresponding author) [1, 2, 3]; Katrin Knittel [1]; Friedrich Widdel [1]; Katie J. Harding [1]; Viola Krukenberg [1, 2]; Dimitri V. Meier [1]; [...], The anaerobic formation and oxidation of methane involve unique enzymatic mechanisms and cofactors, all of which are believed to be specific for C[sub.1]-compounds. Here we show that an anaerobic thermophilic enrichment culture composed of dense consortia of archaea and bacteria apparently uses partly similar pathways to oxidize the C[sub.4] hydrocarbon butane. The archaea, proposed genus Candidatus Syntrophoarchaeum, show the characteristic autofluorescence of methanogens, and contain highly expressed genes encoding enzymes similar to methyl-coenzyme M reductase. We detect butyl-coenzyme M, indicating archaeal butane activation analogous to the first step in anaerobic methane oxidation. In addition, Ca. Syntrophoarchaeum expresses the genes encoding -oxidation enzymes, carbon monoxide dehydrogenase and reversible C[sub.1] methanogenesis enzymes. This allows for the complete oxidation of butane. Reducing equivalents are seemingly channelled to HotSeep-1, a thermophilic sulfate-reducing partner bacterium known from the anaerobic oxidation of methane. Genes encoding 16S rRNA and methyl-coenzyme M reductase similar to those identifying Ca. Syntrophoarchaeum were repeatedly retrieved from marine subsurface sediments, suggesting that the presented activation mechanism is naturally widespread in the anaerobic oxidation of short-chain hydrocarbons.