Johnson, E. M. (Walter Reed Army Institute of Research, Washington, D.C.), Barbara Krauskopf, and L. S. Baron. Genetic analysis of the ViA-his chromosomal region in Salmonella. J. Bacteriol. 92:1457-1463. 1966.-The relative chromosomal location of the ViA determinant, a gene required for Vi antigen expression in Salmonella typhosa (and present also in S. typhimurium), was examined in S. typhimurium x S. typhosa matings. The position of this gene was determined with respect to the histidine (his) and methionine (metG) biosynthesis markers, and to the genetic determinants of somatic antigens 5 (O-5) and 4 (O-4) of S. typhimurium. The gene order established by analyses of the hybrid classes resulting from the genetic crosses was ViA-O-5-metG-O-4 (his). This order suggests that neither ViA nor O-5 is a member of the complex of functionally related structural genes which constitute the O-4 locus. It allows for the possibility, however, of a functional relationship between the genes of the ViA and O-5 loci.