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Phage 13vir is a sheath-tailed phage (Prozesky, de Klerk & Coetzee, 1965, Pl. 1, fig. 5) which arises from single clones of Proteus mirabilis strain 13 (Coetzee & Sacks, 1960). It forms large clear plaques on strain 13. The phage may carry a constitutive mutation in an operator gene which controls development of the prophage in strain 13. It is not inducible with ultraviolet light or mitomycin C but is often present in overnight broth cultures of strain 13. The phage is particularly troublesome in that strain 13 is the host of transducing phage 34 (Coetzee & Sacks, 1960) and lysates of the latter are usually contaminated with phage 13vir. When high titre φ 13vir. 13 (prepared by infecting a broth culture of strain 13) is spotted on a lawn of a locally isolated P. mirabilis strain 5006 the bacteria are killed but serial dilutions of the phage only sporadically form plaques.