Inversions of specific DNA segments have been shown to be involved in regulating gene expression in several systems, and flagellar phase variation in Salmonella is one example. Strains of diphasic Salmonella species, such as S. typhimurium, possess two non-allelic structural genes, H1 and H2, for flagellin, the component protein of flagellar filaments. One or the other is expressed in a bacterial clone. Lederberg and Iino¹ reported that the switch from the expression of one gene to that of the other was controlled by a genetic element linked to the H2 gene and that the state of H2 determined the flagellar phase of the bacterium. Simon et al.^2–4 have shown that the expression of H2 is controlled by a recombination which inverts a region of DNA containing an element necessary for the transcription of H2. We report here that prophages P1 and Mu produce a trans-acting cytoplasmic factor which mediates the inversion of a specific DNA segment in flagellar phase variation of Salmonella.