The short noncontractile tail of P22 allows it to eject its DNA together with three phage-encoded proteins (gp7, gp16, and gp20) into the periplasm of the host. However, the mechanism of phage P22 DNA transport across the cytoplasmic membrane of Salmonella enterica sv. Typhimurium is not known. This process could be mediated by phage-encoded or host-encoded proteins, or a combination of both. Genetic and biochemical approaches failed to identify host factors that are essential for P22 DNA uptake, suggesting that phage-encoded proteins are sufficient to catalyze P22 DNA uptake. This hypothesis was tested by studying the protein-protein interactions, the DNA-binding properties and the membrane partitioning characteristics of the P22 ejected proteins. The energy requirements for P22 DNA transport was also tested by studying the effects of inhibitors of the ATP synthase and uncouplers of the proton motive force. The pull-down assays revealed that the P22 ejected proteins interacted with each other. The gel retardation assays showed that these proteins not only bound DNA nonspecifically but also protected the DNA from degradation by DNase I. The protein gp16 partitioned into the detergent phase of Triton X-114 and into the liposomal fraction. Gp16 was also found to disrupt membranes by possibly forming a channel across the liposomal membrane. When ³²P-labeled DNA was used to detect the transport of DNA inside liposomes, transport of radiolabeled DNA into liposomes was possible only in the presence of gp16 and an artificially-created membrane potential. The requirement for the membrane potential in the transport of phage P22 DNA into the cytoplasm of Salmonella was shown when valinomycin plus potassium chloride reduced the transduction efficiency of the host cells by 63%. The findings reported in this thesis suggest that the transport of phage P22 DNA across the cytoplasmic membrane of the Salmonella host is mediated by the phage-encoded protein gp16, and that transport is driven by the membrane potential of the host. The primary role of the other ejected proteins, gp7 and gp20, might be to protect the ejected DNA from periplasmic nucleases.