In this study, the bacterial. ora of Hyphantria cunea Drury. (Lep., Arctiidae) were investigated during three hazelnut seasons from 1998 to 2000. Four different bacteria were found in dead and living larvae. They were isolated and identified as Bacillus thuringiensis, Escherichia freundii, Micrococcus sp. and Streptococcus sp. Laboratory experiments carried out to determine the insecticidal activities of these isolates showed that E. freundii and Micrococcus sp. did not have any insecticidal effect on second - third instar larvae of H. cunea. However, B. thuringiensis and Streptococcus sp. had 56 and 38% effects, respectively. Crystals and spores from B. thuringiensis were also purified and the crystals, spores and crystals - spore mixture were tested separately against the larvae of H. cunea. It was found that the insecticidal activities of the crystals, spores and crystal - spore mixture were 37.5, 25 and 62.5%, respectively, on second - third instar larvae of H. cunea. These results indicate that the crystal - spore mixture has 6.5% more insecticidal effect than that of the vegetative cells of the B. thuringiensis isolate.