Unrooted phylogenetic tree constructed using a neighbor-joining method. The clones from this study are labeled ARC## and are shown in relation to related species and clones. Brackets indicate clusters UNI (unknown methanogens), Mbr (Methanobrevibacter ruminantium related clones), and Mss (Methanosphaera stadtmanae related clones). Bootstrap values greater than 70% for 1000 trees are shown at major branch points. Halobacterium halobium was used as the out-group. The bar represents a sequence divergence of 10 %.