The basic step for establishing the causal agent of a disease involving microorganisms is the application of Koch's postulates. The aim of this work was to confirm that the mortality of Myzus persicae (Sulzer) (Hemiptera: Aphididae) was due to Bacillus thuringiensis Berliner (Bacillales: Bacillaceae) strains (GP780, GP139, GP209, GP528, GP782, GP300, GP777, and GP402) and to document the signs of infection caused by 4 of these strains (GP300, GP528, GP402, and GP77). Our results suggest that the strains that were tested in the bioassays were responsible for the observed aphid mortality and that the physical changes were induced by infection with B. thuringiensis.