Abstract: In the present study , the 16S rDNA of Bif idobacterium inopinatum with low GC content and B . thermacidophilum sp. nov. was sequenced. A phylogenetic dendrogram was constructed using the two sequences and other 16S rDNA sequences of 19 Bif idobacterium spp. and eight related genera obtained from DNA databases. The results showed that except for B . inopinutum , all the bifidobacteria were clustered at similarity levels( ≥92 %) of 16S rDNA sequences. Although B . inopinutum showed a relatively lower similarity (87 %～89 %) with the other bifidobacteria , it was closely related to the Bif idobacterium spp. rather than to other gram-positive bacteria with similar GC content . This study did not reveal the evolutionary pattern from low GC to high GC content in the gram-positive bacterial phylum. In addition , the divergence between 16S rDNA sequence and genomic DNA GC content in the study of bacterial phylogenetics has to be resolved using other conserved macrobiological moleculars.