Prophages P1 and Mu produces a trans-acting factor possessing the din+ activity which catalyzes the inversion of the specific DNA segment responsible for flagellar phase variation of Salmonella, din mutants were isolated from PICMclr100 phage by selecting phages that did not suppress the yh2 mutation of Salmonella in prophage state. No inversion loop structure was detected among DNA forms arising after denaturation and rehybridization of DNAs extracted from the din mutants. The DNA fragment containing C region of P1 was cloned on a plasmid vector, pCR1. The resulting hybrid plasmid, pKK2, was shown to possess the din+ activity: the vh2 mutant of Salmonella harboring the plasmid changed the flagellar phase. From analysis of the plasmid by use of BamHI and Bgl II, the din gene specifying the din+ activity was located near or within the C region of P1. It is highly plausible that the din gene of P1 was also involved in the inversion of the C region. Similarly, the DNA fragment containing the G and beta segments of Mu was cloned on pCR1. The resulting hybrid plasmid, pII101, also possessed the din+ activity.