ABSTRACT A transposon-based, genomewide mutagenesis screen exploiting the killing activity of a lytic ViII bacteriophage was used to identify Salmonella enterica serovar Typhi genes that contribute to Vi polysaccharide capsule expression. Genes enriched in the screen included those within the viaB locus ( tviABCDE and vexABCDE ) as well as oxyR , barA/sirA , and yrfF , which have not previously been associated with Vi expression. The role of these genes in Vi expression was confirmed by constructing defined null mutant derivatives of S . Typhi, and these were negative for Vi expression as determined by agglutination assays with Vi-specific sera or susceptibility to Vi-targeting bacteriophages. Transcriptome analysis confirmed a reduction in expression from the viaB locus in these S . Typhi mutant derivatives and defined regulatory networks associated with Vi expression.