Phage P1 does not adsorb to S. typhinurium wild type cells. It does adsorb to rough derivatives including strains with mutations in the galE gene. Phage strain P1CM clr-100 can be efficiently propagated in S. typhimurium derivatives, either by induction of a lysogene, or by lytic infection. Phage P1 lysates are able to mobilize genetic markers in a generalized fashion. The transduction system is essentially identical to that in Escherichia coli, except that CaCl2 is not required for efficient adsorption. Two regions of the S. typhimurium chromosome were mapped by P1-mediated transduction. Several examples of genes linked by P1, and unlinked by P22, are presented. The relative efficiency of P1 over P22 in transduction was not determined, however. Data presented indicate unambigously that the gene order for the trp region is: his ... dad A-hem A-trp-pyrF ... pyrC but known markers in between were not used. The gene order for the cys A region was determined to be as follows: pheA ... purC-cys A-trz A-pts-dsd-aro D-purF ... his, and special mapping problems for this region are discussed.