THE 3,000 base pair-long G segment of temperate coliphage Mu DNA has been shown to invert spontaneously with high frequency1,2. Inversion, which is independent of the rec system of the host3, results in Mu phage with the G segment in either orientation. Nearly equal numbers of both G orientations are observed in a lysogen or when a lysate is obtained by induction of a lysogen. When phage is grown by a series of infections one and the same G orientation prevails4. One attractive explanation for these observations implicates inversion of the G segment in the control of phage or host functions. We report here experiments which show that the orientation of the G segment determines infectivity of Mu phage particles and that Ginversion is mediated by a phage-specific enzyme system.©1978 Nature Publishing Group.