Cellular fatty acid (CFA) composition of Bacillus thuringiensis var. kurstaki (Btk) preparations was determined by use of the MIDI Sherlock microbial identification system on a Hewlett-Packard 5890 gas chromatograph. Four commercial preparations--one Btk sample obtained from the U.S. Forest Service, one Btk sample obtained from Dugway Proving Ground, and Btk and Bacillus thuringiensis var. israelensis (Bti) preparations obtained from American Type Culture Collection (ATCC)--were analyzed and evaluated. This study demonstrated the capability to detect the strain variation in the bacterial species Btk and to clearly differentiate strain variants on the basis of qualitative and quantitative differences in hydrolyzable whole CFA compositions in the preparations examined. We conclude that CFA analysis may be used to identify commercial products but that a more intensive study would be required to evaluate the potential of CFA to provide an inexpensive screening tool applicable to several levels of isolate or product evaluation, including how applied preparations might interact with natural populations over time.