Numerous virulence factors such as O antigen have been proposed to account for the fulminating and destructive nature of V. vulnificus infections. To better characterize the role of O antigen, a pmm gene encoding a phosphomannomutase was identified and cloned from V. vulnificus. The deduced amino acid sequence of the pmm was 42 to 71% similar to that reported from other Enterobacteriaceae. Functions of the pmm gene in virulence were assessed by the construction of an isogenic mutant, whose pmm gene was inactivated by allelic exchanges, and by evaluating its phenotype changes in vitro and in mice. The disruption of pmm resulted in a loss of more than 90% of phosphomannomutase, and reintroduction of recombinant pmm could complement the decrease of phosphomannomutase activity, indicating that the pmm gene encodes the phosphomannomutase of V. vulnificus. There was no difference in the $LD_50S$ of the wild-type and the pmm mutant in mice, but the $LD_50S$ observed by the mutant complemented with recombinant pmm were lower. Therefore, it appears that PMM is less important in the pathogenesis of V. vulnificus than would have been predicted by examining the effects of injecting purified LPS into animals, but it is not completely dispensable for virulence in mice.