The closer proximity of Frankia and Acidothermus cellulolyticus relative to the morphologically close Geodermatophilus found previously was confirmed by resequencing the rrs gene of Acidothermus cellulolyticus and the housekeeping gene, recA. The diagnostic sugar 2-O-methyl-D-mannose was detected only in Frankia, while hopanoid lipids were present at high levels in both Acidothermus and Frankia.