AbstractSerratia marcescens HYbacteria must be lysogenic with either prophageyor ψ to make it possible for phage χ to form plaques unless they carry a so‐calledinkmutation. Genes inyand ψ termedanyandanpwere identified that after infection ofink+cells are necessary for an effective propagation of these phages as well as of coinfecting χ phage. When χ infectsyand/or ψ‐Iysogenic cells it transactivates the respective prophage genes by means of two early genes termedtayandtap.It appears that on infection of nonlysogenicink+cells χ damps its own development, provided the regulatory region of the responsible gene is undermethylated. After χ infection duly to achieve the special methylation of this region seems to be the function ofanyandanp.There are some more genes inyand ψ prophage under the control oftayandtap, concerning in both cases a Dam methylation (recognition sequence GATC) of χ DNA, a recombination proneness under restricting conditions of χ DNA not modified by the modification enzyme ofHY, and the χ plaque size. By hybridization studies a region of homology common toyand ψ was demonstrated which from its size might comprise all the transactivated genes. The view is supported by genetic data indicating an affinity among theanyandanpgenes.Investigation of variousanymutants were indicative of DNA inversions in this region of theygenome. Surprisingly some of theanymutants had become sensitive in their plaque forming ability to an inhibitory activity exerted by prophage ψ. Mutants of ψ unable to interfere but still able to lysogenize were isolated. A model is presented accounting for the formation of pleiotropic and nonpleiotropic mutations with Any phenotype and their reversion types. Possible functions of theygenes and their counterparts