The cellular fatty acid composition of Bacillus thuringiensis ssp. israelensis (B.t.i.) ONR-60A was determined by gas-liquid chromatography (GLC). Isolates of ONR-60A were obtained from the Bacillus Genetic Stock Center, Pasteur Institute, and USDA culture collections. These isolates were compared to one another and the culture collections differed in the strains that they contained. Eleven separate strains of ONR-60A were identified, based on fatty acid composition of the cell envelope. The current Pasteur Institute international standard for B.t.i., IPS 82, was also analyzed and it consisted of 3 strains using these same criteria. Profiles were created for each strain found in ONR-60A and IPS 82, and validated against profiles of commercially produced B.t.i. from 2 time periods, 1983-84, and 1990-94. Approximately 84% of the commercially produced B.t.i. (134 out of 160 agar plates) was identified as belonging to the strains found in ONR-60A and in IPS 82. The most prevalent strain found in IPS 82 is also the most prevalent strain in the recently produced commercial material, 40.8% of the commercial plates analyzed, but commercially produced B.t.i. also retained a strong ONR-60A ancestry (35% of the plates analyzed).