Abstract: In recent years, there have often been serious withering syndrome in many breeding farms of Haliotis diversicolor Reeve in South China. It is infectious to all sizes of abalones with high mortality. The symptoms of sick abalones are: mantle and visceral mass shrink and become lethargic, weak in adhesion and finally die. A bacterial strain WS was isolated from the tissues of cultured H.diversicolor with withering syndrome. Artificial infection test proves that it has strong pathogenicity to healthy H.diversicolor and has symptoms similar to those which get sick spontaneously. We carried out a morphological observation and a test on the physiological and biochemical characteristics of strain WS, which indicates that it is most similar to Vibrio harveyi. In order to define its phylogenetic position, we sequenced, analyzed and compared the 16S rDNA genes whose partial fragments were amplified by a pair of universal primers, and finally constructed a molecular phylogenetic dendrogram based on the genetic distance. The result shows that strain WS and V.harveyiAY750577 group into one cluster, with a 99.70% homology. According to morphological, physiological, biochemical and phylogenetic analyses, strain WS is proved to be V.harveyi.