Chlamydia abortus(C.abortus)is an obligate intracellular bacterium responsible for producing disorders in domestic and wild mammals.Recently,the yak(Bos grunniens)population has exhibited reproductive disorders,which are considered to be associated with C.abortus in Qinghai,China.Therefore,this study was conducted to identify C.abortus by PCR and culture in aborted yaks.During the birth season of 2012,a total of 9 aborted fetuses(each from a different herd)and 126 vaginal swab samples from the 9 herds were collected and analyzed.C.abortus DNA was detected from all of the 9 aborted fetuses and 30 of the 126 vaginal swab samples(23.81%)from yak cows in the selected herds.4 C.abortus strains were isolated from embryonated egg yolk sacs inoculated with foetal organ suspensions.In order to differentiate from C.psittaci and C.pecorum,the isolated C.abortus strains were further identified by PCR-based RFLP analysis.All isolates showed identical restriction profiles with the C.abortus reference strain using AluI restriction enzyme in the RFLP test.Moreover,the isolated C.abortus strains and C.abortus-positive vaginal swab samples were genotyped by multi-loci variable number of tandem repeat analysis(MLVA)and all were belonged to the genotype 2 group.These findings suggested that C.abortus played a substantial role in yak abortion in Qinghai,China.Chlamydia abortus(C.abortus)is endemic in ruminants throughout the world.C.abortus can efficiently colonize the placental trophoblasts and is one of the causative agents of abortion and foetal loss in sheep,goats and cattle in many countries [1-3].