Selection of Bacillus thuringiensis strains with insectidal activity against armyworm (Plutella xylostella) and diamondback moth (Heliothis armigera) NguyÔn ThÞ Hoµi Hµ1 , Ng« Giang Liªn2 Summary Over 20 species of natunally occurring insect - specific bacteria have been isolated from the soil, insects and plants, but only a few have been studied intensively. Much attention has been given to Bacillus thuringiensis, a species that produces microbial insecticed toxic to specific groups of insects. Insecticidal activity of four strains of Bacillus thuringiensis from the Museum of the Center of Applied Microbiology has been tested against Plutella xylostella and Heliothis armigera. Keywords: Bacillus thuringiensis strains, insectidal activity, Plutella xylostella, Heliothis armigera.