When bacteria lysogenized by certain temperate phages like λ and p2 are superinfected by distinguishable mutants of their prophage, the superinfecting phage appears in the progeny released after induction of vegetative phage growth (1, 2). Phage p22 differs, however, for although bacteria lysogenized with it adsorb superinfecting phage p22, no superinfecting genetic markers like c 2 (clear plaque) or h (host range) appear in the phage obtained after induction, whether spontaneous or produced by u.v. irradiation. Superinfecting phage p22 is thus excluded and the wild-type prophage may be termed ‘excluding’ (x +). Non-excluding (x) mutants have now been isolated by taking advantage of the phenomenon of lysogenic conversion. Salmonella typhimurium only forms somatic antigen 1 (01) if it is infected by phage p22 possessing the converting gene a1 + (3).