Aims: To identify and characterize Bacillus thuringiensis strains highly toxic to Spodoptera frugiperda, and to explore the genetic diversity of such strains. Methods and Results: The insecticidal activity of 1100 strains of B. thuringiensis from Colombian soil samples was assayed against ®rst instar S. frugiperda larvae, and 32 active strains were found. After a second bioassay evaluation, the eight most potent strains were selected for further characterization, which included crystal protein pro®les determined by polyacrylamide gel electrophoresis, plasmid pro®le, plasmid restriction patterns, cry gene composition, qualitative determination of b-exotoxin production, random ampli®ed polymorphic DNA, serotyping, and toxicity to S. frugiperda. All Colombian strains contained cry1Aa, cry1Ab, cry1Ac, cry1B, cry1C and cry1D genes. However, PCR pro®les of the Colombian strains suggested the presence of variants of the cry1 genes. Serotyping indicated that these strains belong to the kurstaki, thuringiensis, canadiensis and indiana subspecies. Interestingly, three strains belonging to different serotypes and subspecies were found in the same soil sample, and toxicity ranged between 11 and 976 ng cm ±2 of diet. Conclusions: It has been shown that B. thuringiensis strains belonging to different serotypes and displaying variable potency to S. frugiperda larvae can be found in the same soil sample. Signi®cance and Impact of the Study: The results obtained indicate that some of the B. thuringiensis strains studied could be of interest for further development for S. frugiperda control programmes.

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