During August and September of 2013,a serious infectious disease characterized by body ulcer,internal organs haemorrhage and nodules showing on epicardium occurred in Acipenser baerii in Hanyuan Lake,Ya'an,Sichuan Province.To elucidate the aetiology,the bacteria isolated from the livers,spleens and kidneys,and artificial infection,physiological and biochemical analysis and molecular analysis of the isolate were conducted.The results showed that a Gram-positive,chain-forming coccus(Ab130920)was isolated and the strain was lethal to A.baerii.Physiological and biochemical characteristics were consistent with Streptococcus iniae(ATCC29178);the 16S rRNA gene sequencing analysis by Blast in GenBank indicated that the isolate(KJ162337)showed high levels of similarity to S.iniae(99%); A phylogenetic tree was constructed based on the 16S rDNA sequences of the isolate and the homologous sequences of other strains of Streptococcus,and the strain and other S.iniae strains formed a tight cluster; Meanwhile,expected segment of 870 bp was yielded in the specific PCR detection of S.iniae based on S.iniae lctO gene.As expected,the strain Ab130920 was positive by multiplex PCR for all five virulence genes(cpsD,simA,sagA,pdi and scpI)test,showing amplification products of the expected sizes,which was further identical the isolate to S.iniae and confirmed the pathogenicity of it,and consistent with the result of artificial infection.The drug sensitivity results showed the isolate was sensitive to amoxicillin,doxycycline and florfenicol,etc. but resistant to novobiocin,ofloxacin,rifampicin.