Mud-P22 derivatives are hybrids between phage Mu and P22 that can be inserted at essentially any desired site on the Salmonella chromosome (Benson and Goldman, 1992; Youderian et al., 1988). Induction of Mud-P22 insertions yields phage particles that, as a population, carry chromosomal DNA from the region between 150 and 250Kb on one side of the insertion. Thus, phage lysates from a representative set of Mud-P22 insertions into the S. typhimurium chromosome yield an ordered library of DNA that provides powerful tools for the genetic and physical analysis of the Salmonella genome. Although Mud-P22 has not yet been used in other species, this approach should be applicable in a variety of other bacteria as well.