Among Bacillus thuringiensis strains, strain T84A1(serotype 4a, 4b, var. dendrolimus) has been demonstrated as a lysogenic strain when strain T84A1 was tested on strain S-16(serotype 4a, 4b var. dendrolimus; acrystalformer). Temperate phages were not induced from strain T84A1 by ultraviolet, hydrogen oxide or mytomycin C treatment. However, the temperate phages were produced by a mixed culture in the following combination of bacterial cells.