Segmented filamentous bacteria (SFB) have been found in the alimentary tracts of a wide range of animals, where they activate the mucosal immune system. We determined the 16S rDNA sequences of SFBs derived from crab-eating monkey and Fischer 344 (F344) rat, in addition to of that from BALB/c mice (Umesaki et al, 1995), and compared these sequences with those of SE (Swiss) mice, WU rats and chickens reported by Snel et al. (1994, 1995), with special reference to inter-species and within-species differences of the host animals. SFBs isolated from six different hosts mentioned above exhibited levels of 16S rDNA sequence homology of more than 97%. There was only slight divergence in the 16S rDNA sequence between BALB/c and SE mice, and F344 and WU rats, 0.1 and 0.4%, respectively. However, there was some inter-species divergence, except between monkey and rats, suggesting that SFB isolated from different species of animals belong to different species, as proposed by Snel et al. (1995). The phylogenetic tree of SFB 16S rDNA derived from these six kinds of animals, including crab-eating monkey, does not necessarily suggest the coevolution of SFB and their host animals.