Several B. thuringiensis serotypes are used for the production of bacterial insecticides. The important ingradient of bacterial insecticide produced by B. thuringiensis is a parasporal crystal protein formed during sporulation. The bioinsecticide containing several B. thuringiensis serotypes have great potential in the control of certain insects, because of their high selectivity and the absense of any harmful effect on humans, plants, animals, polinators and predators. But the generation of cry- (non-virulent or acrystalliferous) strains during the growth in the medium are relatively high without mutagenic treatments. Plasmids can play role in this area. Because plasmids have been detected in Bacillus thuringiensis strains belonging to several serotypes and have not been found in certain cry- varyants. It is concluded that parasporal crystals are plasmid determined in B. thuringiensis varyants.