Bacillus thuringiensis, the so called Bt, is a Gram-positive, soil-dwelling bacterium. Bt is commonly used as a biological pesticide, alternatively, the Cry toxin may be extracted and used as a pesticide. Bt also occurs naturally in the gut of caterpillars of various types of moths and butterflies, as well on leaf surfaces, aquatic environments, animal feces, insect rich environments, flour mills and grain storage facilities.

References:

1. http://en.wikipedia.org