Strains of Bacillus thuringiensis lysogenic for temperate bacteriophage Tm2 inhibit the development of virulent bacteriophage Tg4, although the latter can absorb to and kill these bacteria. Tm2 mediated interference did not act directly on the Tg4 genome; rather, the phage altered the host physiology. The latent period of Tg4 growth in Bac. thuringiensis lysogenic for phage Tm2 was longer than when nonlysogenic bacteria were infected. Progeny yields have been counted in Bac. thuringiensis strain mixedly infected with two unrelated phages-Tg4 and Tm2 in a varying order of addition. The dominance was dependent on the order of addition. Under the conditions of mixed infections, only single phage (Tm2 or Tg4) produced a significant number of progeny, provided, it was added 5 min before the infection with the other phage. When two phages were added simultaneously, the yields of both phages were reduced.