The virulence factors of Burkholderia pseudomallei, the causative agent of melioidosis, are not fully understood. We have identified a gene with homology to the Salmonella typhimurium mouse virulence gene, mviN, a member of the mouse virulence factor family. Expression studies with an insertional mutant containing a lux operon demonstrated that the expression of the gene is influenced by free-iron availability in the media and by growth phase. The mutant displayed an increased LD50value in the hamster infection model and a loss of the ability to invade human lung epithelial cells. The mutant has a slower growth rate than that of the wild type. Both defects were restored to various degrees when complemented in trans with the mviN gene. The mutant contains an insertion at 1229 bp of the 1548 bp gene, resulting in a truncated protein that is presumably responsible for the defects. Deletion mutants of the entire B. pseudomallei mviN gene were obtained only in the presence of the complement vector. This result and the inability of the complemented deletion mutant to lose the plasmid in the absence of antibiotic selection suggest that the gene is essential to B. pseudomallei.Key words: Burkholderia pseudomallei, mviN, virulence, essential gene.