A bacteriophage, øC2(W), which attacked Streptococcus lactis strains C2, ML3 and 712, is described. It had a prolate head and a non-contractile tail and produced large haloes around plaques. Infection of eight paired-strain cultures, each containing Str. lactis C2, with øC2(W) resulted in marked inhibition of acid production for six cultures. Direct phage–nonhomologous ‘host’ contact was not required for inhibition. Lysates of øC2(W) contained a phage-induced lysin. Evidence is presented that the inhibitory effect of øC2(W) against paired and multi-strain cultures is due to the lysis of phage-insensitive strains by phage lysin. An isometric phage, ø712, was shown not to produce such a lysin.