Bacillus thurlngiensis (Bt) is a bacterium that belongs to the genus Bacillus. It is pathogenic to insect larvae, mainly Lepidoptera species. Several insecticidal toxins can be produced by the different strains of Bt. The most important one is the crystal (delta-endotoxin) which, alone or in conjunction with the spore, will kill the insects. Parasites and predators of Lepidoptera and beneficial insects are generally unaffected by Bt. In the U.S. commercial products of Bt are produced with the HD-1 strain and contain only the spore and crystal as their entomopathogenic ingredients. To be effective these must be ingested. Therefore timing of the application and thorough coverage of the treated foliage are important. Bt can be applied by conventional means. Highly alkaline water should not be used for mixing and excessive heat during storage should be avoided. A brief listing of Bt formulations for gypsy moth control is included.