SUMMARY The genetic properties of a new (hspS) host specificity of Salmonella typhimurium were investigated using bacteriophage L. Phage L is a better substrate for s-specific restriction than phage P22. Mutants deficient in s-restriction only were found at the same frequency as mutants deficient in both restriction and modification. Crosses between S. typhimurium Hfr and S. typhimurium F- or between Escherichia coli and S. typhimurium showed that the s system has the same chromosomal location as the K system of E. coli. The s system was introduced into E. coli and found to be effective on phage A.