Portunus trituberculatus,mainly distributed in East Chins Sea and Yellow Sea,is a large marine economic crabs with rich nutritional and medicinal properties.In recent years, P.trituberculatus aquaculture has developed rapidly along the Jiangsu coastal waters,while some epidemic diseases broke out which was attributed to many factors,for example,pathogenic microorganism.In the late autumn of 2009,the cultured P.trituberculatus suffered from serious disease in one farm of Ganyu County in Jiangsu Province.The main symptom displayed as drumble,not feeding,the tissue oedema of liver,gill and muscle;the liver and muscle were rotten in some crabs.Virtually pure dense cultures were obtained from liver,muscle,and lymph fluid of diseased P.trituberculatus,and strong pathogenicity of the isolate strain(JG0911201)to the P.trituberculatus.The phenotypic characteristics of isolate strain were examined,including morphological characteristics,physiological,and biochemical characteristics,the 16S rRNA and gyrB genes were amplified and compared with those sequences deposited in databases.Phylogenetic analysis of 16S rRNA and gyrB genes indicated high homogeneity between the isolate strain and Citrobacter freundii from GenBank database.The 16S rRNA gene fragment length is 1 451 bp(GenBank accession number HQ170626)and gyrB gene fragment length is 1 186 bp(GenBank accession HQ170627)The results showed that the identified isolate strain belonged to C.freundii.Colonization factor antigen was a pathogenic factor of enterobacteriaceae which could produce enterotoxin that was capable of causing death.And the cfa gene(100 bp)could be amplified by PCR.It was concluded that the isolated strain C.freundii carried cfa and provided an effective method for detecting the C.freundii.C.freundii was a conditional pathogen that is gramnegative,bacillus,aerobic or amphimicrobian.And many diseases were attributed to the C.freundii for man,reptiles,amphibians.In recent years,a lot of aquatic animals such as Cherax quadricianalus,Ophicephalus argus,Oreochromis niloticus,Eriocheir sinesis etc.,were also infected by the C.freundii.In order to choose the appropriate antibiotic the drug sensitive was tested,and testing results by paper extension methods for drug sensitive showed that among 48 antibiotics,ciprofloxacin,fleroxacin,ceftriaxone,enoxacin,enrofloxacin,floxacin and levofloxacin had the most inhibitive effects on the strain.The paper reported for the first time that C.freundii could lead to the mass mortality of P.trituberculatus in Jiangsu coastal waters and a detailed study of the pathogen was made,which would be helpful in the disease control and health management in P.trituberculatus cultivation.