1. The atypical citrate-utilizing ability to two strains of E. coli has been shown to be plasmid-encoded. Strain V414 carries a 130 Mdal conjugative Cit+ plasmid that also specifies Tcr and Cmr. Strain V517 carries 9 different plasmid species but only the 36 Mdal species is correlated with Cit+ ability. These plasmids are different from previously reported Cit+ plasmids of E. coli and Salmonella, which express thermosensitive conjugal transfer systems. 2. A 9 kb Pstl fragment, carrying the Cit+ genes of pWR60, has been cloned into the pBR325 plasmid. 3. Metabolic studies indicate that intact citrate is not incorporated directly into whole cells. Rather, atypical citrate utilization by these E. coli strains appears to involve partial metabolism of citrate at the cell surface before or during uptake. 4. The expression of atypical Cit+ ability by the parental pWR60 plasmid or by the recombinant pWR61 plasmid appears reversible and may involve an expression switch mechanism (i.e., insertion sequence element). 5. Two widely separated genetic loci, viaA and viaB, are necessary for Vi antigen synthesis in Salmonella and Citrobacter. In some strains of C. freundii, Vi antigen expression is reversible, a phenomenon which can be visualized by a colonial morphology transition between Vi-expressing and -nonexpressing forms. 6. The C. freundii viaB locus appears to encode the Vi antigen as well as the genetic "switch" mechanism controlling reversible Vi antigen expression. The viaA locus, which is found in several different bacterial species, may encode some common property (e.g., cell surface structure or enzymatic activity) that is needed for Vi antigen expression. 7. S. typhi and E. coli K12 hybrid strains which carry the C. freundii viaB locus have been constructed. These hybrid strains express reversible Vi antigen expression, even in the absence of general recombination (i.e., functional recA gene product). 8. The C. freundii viaB locus was transposed via Mu-mediated events to an F'lac plasmid in the E. coli K12 hybrid strain WR2376. F' plasmids carrying the viaB locus should serve as a highly enriched source of viaB DNA for physical examination of the switch mechanism. 9. Genetic manipulations such as those described herein can be used to study virtually any plasmid, viral, or chromosomally-encoded property. The resultant better understanding of biochemical pathways and of genetic regulatory control systems, and the isolation of desired gene sequences should provide ample information and materials for improving chemical processes and constructing vaccines against various organisms.