1,490 nucleotides of the 16S rRNA gene of a Gram-negative, thermophilic and gliding bacterium, Thermonema lapsum, have been sequenced. Phylogenetic analysis indicates that T. lapsum is related to cytophaga-flavobacteria-bacteroides (CFB) and is confirmed by the identification signature nucleotides that define this group. Further phylogenetic analysis indicates that T. lapsum forms the deepest branch in the CFB group; this observation was confirmed by the identification of unique nucleotide and nucleotide pairs which separate T. lapsum from all other members of this group. The phospholipid fatty acid (PLFA) profile also confirmed that T. lapsum is related to the cytophaga-flavobacteria-bacteroides group and also to selected members of the genus Flexibacter; the PLFA profile is unique to T. lapsum.