SummaryWhether infection by bacteriophage P2 results in lysogenization of the host or vegetative growth of the phage depends upon a race between transcription from the repressor promoter Pc and the early promoter Pe; transcription from these promoters is mutually exclusive, since the Pcrepressor Cox is formed from the Petranscript and the Perepressor C from the Pctranscript. The involvement of integration host factor (IHF) in the lysogenization ofEscherichia coliK12 by P2 was tested by comparing wild‐type and IHF‐deficient (him Aandhim D)mutants. No lysogenic clones were formed following infection of the mutant bacteria. A switch plasmid that contains Pc‐C‐car andPe‐cox‐kanwas used to test the choice for expression of Pc versus Pe. In the wild‐type K12 bacteria, 20% of the clones expressed Pe transcription and 80% Pc transcription, whereas all transformed IHF‐defective clones expressed transcription from Peonly. The effects of IHF on thein vivoexpression of the Peand Pcpromoters were only marginal. The IHF protein was found to bind upstream of the Pepromoter, where a potentialihfsequ