The Formosan subterranean termite, Coptotermes formosanus Shiraki, was studied for its ability to suppress two entomopathogenic bacteria, Bacillus thuringiensis subspecies israelensis (Bti) and thuringiensis (Btt). Different group sizes (50, 25, 10, and no termites [control]) of C. formosanus were placed on well-grown Bti or Btt agar plates. On day 1, the diameters of Bti and Btt colonies in the three treatments containing termites were significantly smaller than in the controls. The diameters of Bti and Btt colonies in the 50-termite treatment were significantly smaller than in the 10-termite treatment. However, neither was significantly different from the 25-termite treatment. This group sized dependent suppression was even more distinct on day 2. On day 5, inhibitory zones were observed in all three treatments containing termites where Bti or Btt colonies originally grew. The Bti and Btt cells from these inhibitory zones regenerated on new plates after transfer from 25- and 10-termite treatments as did the controls, but no regeneration was observed after transfer from 50-termite treatment. Results show that the presence of C. formosanus can suppress the growth of Bti and Btt and the suppression effect enhanced with increased of group size. Moreover, antagonistic tests show that natural bacteria carried by termites play a role in the suppression of Bti and Btt.