The summer of 2002 was hot and dry in Xiamen, and the seawater salinity was higher than usual, sometimes reaching 38 in Tongan Bay. From the end of May , ulcer disease occured on a large scale in Tongan Bay. In the middle of June, a heavy rain brought about high mortality among the cultured groupers, which caused serious loss. The characteristics of ulcer disease were observed. Infected groupers displayed sluggish swimming and refused feeding, several days later, the groupers.. eyes swelled and became ulcerated, the tail turned red, the scales became detached and the back muscle gradually ulcerated, then the eyes even dropped out and the muscle became so necrotic that the spine was exposed, finally the diseased f ishes died. Anatomized the dead fish and found that the livers and gills were pale and the gallbladder was distended. The time course f rom appearance of disease signs to death lasted about a week. Dominant bacteria, which were Gram-negative and seen short rod w ith single polar f lagellum under electron microscope, were isolated and designated TS-628. In artif ical infection test all f ish of the experimental group died on the third day after injected with bacterial suspension, while all the fish in control group showed no signs till the end of another week observation. The dead grouper had pale livers and gills and small ulcerations on the caudal fin and anal fins. These were the same signs as the natural infected f ishes. The re-isolate also had the same characteristics as TS-628, which proved the isolate TS-628 was the pathogenic bacteria that triggered this ulcer disease. Different methods were used to ident ify the pathogenic bacteria in this study. The identification result through VITEK-AMS system GNI card was that TS-628 was Burkholderia mallei , with 99% reliability. While traditional biochemical identification revealed that TS-628 exhibited relatedness to Vibrio harveyi through comparison with Bergey.. s Manual Determinat ive Bacteriology. In order to confirm absolutely different results above, further researches were carried out to identify TS-628 again. So 471bp sequence of TS-62816S rRNA gene was amplify and compared with all DNA sequences ( 1192858 in total ) in genebank ( GenBank EMBL DDBJ PDB) , homology analyses showed that 16 sequences were picked out to have the highest similarity , with 98% identity. These 16 sequences all originated from vibrio, 8 of them didn.. t mention specific names, 6 of them were V. harveyi and 2 of them were V. carchiariae which also belongs to harveyi because their similarity in physio logical and biological characteristics and DNA homology. According to morphological features, physiological and biochemical characteristics and 16S rRNA gene homology comparison of the bacteria, the pathogenic bacteria were V. harveyi . Drug sensitivity test showed that the pathogenic bacteria were highly sensitive to chloramphenicol, actinospectcin etc. V. harveyi is a kind of pathogenic bacterium commonly found in marine-fishes culture, but this is the f irst time that V. harveyi is reported as the pathogenic bacteria of cage-cultured grouper in China. And V. harveyi should be regarded as an opportunistic pathogen which has close relat ion to temperature and salinity and easily causes vibriosis under conditions of high temperatures and drought. Therefore, it is necessary to guard against V. harveyi vibriosis in such summer days.