Abstract: Objective To identify an unknown strain,with similar clone to Clostridium difficile（C.difficile） by sequencing and phylogenetic analysis,and santimicrobial susceptibility. Methods C.difficile selective medium was used to isolate the strain from fecal samples of a healthy pet dog.To identify the unknown strain,a C.difficile latex agglutination rapid test kit was used and triosephosphate isomerase（tpi） gene was detected by PCR.The 16 S rDNA of the unknown strain was sequenced and phylogenetic tree was constructed to analyze its evolution.Agarose dilution method was used to evaluate the sensitivity of the strain. Results The unknown strain was isolated using C.difficile selective medium.Neither tpi gene nor C.difficile antigen was found,which indicated that the unknown strain was not C.difficile.According to 16 S rDNA sequence alignment and comparison and phylogenetic analysis,the strain of interest was Clostridium sordelli（C.sordelli）,with a relative close evolutionary relationship with C.difficile and sensitive to ampicillin,clindamycin and moxifloxacin. Conclusions The unknown strain isolated from the healthy pet dog is C.sordelli.With a relative close evolutionary relationship with C.difficile,the strain’s antimicrobial resistant pattern greatly differs from C.difficile.