A method, which we call localized mutagenesis, is described for the isolation of temperature-sensitive and other types of mutations in any specific small region (about 1%) of the bacterial chromosome. The principle of this method is to mutate the transducing DNA rather than the bacterial DNA. One can select for the introduction of this mutated DNA into any particular region of the bacterial chromosome by transducing an auxotrophic marker in that region to prototrophy, thereby introducing new mutations in the neighborhood. We have used this method to isolate many different temperature-sensitive mutations in genes of unknown function in particular regions of the chromosome. Since the method is very simple, it can be used to saturate any region of the map with mutations in essential genes, or for various types of genetic manipulations. Although we have used hydroxylamine-mutagenized phage P22 and Salmonella typhimurium, the method should be applicable to other mutagens and bacteria and transducing phage.