P22 transduction of chromosomal genes from Salmonella typhimurium into Salmonella typhi occurs at a low frequency. Transduction of plasmids from S. typhimurium into S. typhi occurs at a frequency similar to that between S. typhimurium strains, indicating that the barrier to transduction of chromosomal genes is not due to an inability of P22 to inject DNA into S. typhi or a restriction endonuclease that rapidly degrades foreign DNA. Furthermore, transduction of mutS and mutL derivatives of S. typhi with chromosomal genes from S. typhimurium occurs efficiently. These results indicate that the transduction barrier is due to activity of the recipient mismatch repair system, which senses sequence divergence and disrupts heteroduplexes in favor of recipient sequences. Inactivation of the mismatch repair system allows P22 transduction to be used as an effective tool for constructing S. typhi-S. typhimurium hybrids.