The phylogenetic position of an anaerobic, non-spore-forming thermophile ‘Thermobaculum terrenum’ was investigated on the basis of gene order data from completely sequenced bacterial genomes. Gene order data can be an excellent source of phylogenetic information. Shared gene arrangements are unlikely to have arisen by chance convergence. They are likely to reflect common ancestry. ‘Thermobaculum terrenum’ was found to share three gene arrangements that are present uniquely in genomes of members of the phylum ‘Chloroflexi’, indicating convincingly that ‘Thermobaculum terrenum’ is a member of this phylum. Branching orders within the phylum ‘Chloroflexi’ were inferred by identifying monophyletic groups of species, which were circumscribed by characteristic gene arrangements. The branching orders thus inferred were in good agreement with previously reported phylogenies based on single 16S rRNA gene sequences and on multiple protein sequences. The gene order comparisons revealed a close phylogenetic affinity of ‘Thermobaculum terrenum’ to Sphaerobacter thermophilus and Thermomicrobium roseum.