Locations of six Tn10s, closely linked to each of the six IS200s on the genomic cleavage map of Salmonella typhimurium LT2, were determined by digestion with XbaI and BlnI and separation of the fragments by pulsed-field gel electrophoresis; the locations were then further defined by P22-mediated joint transduction. The orientation of each IS200 with respect to its linked Tn10 was determined by Southern blotting. The locations of IS200-I, IS200-III, and IS200-V were confirmed to be close to sufD, melB, and purC, as previously indicated. IS200-II is jointly transduced with cysG. IS200-IV is near fliA; the linked Tn10 is inserted in fli, making the strain nonmotile. IS200-VI is jointly transduced with aspC but not with aroA. IS200 is transposed to a seventh site in some strains, while remaining in the other six locations described above. These data indicate that genome analysis by pulsed-field gel electrophoresis can locate the positions of Tn10s with accuracy sufficient to predict P22-mediated joint transduction.