Melolontha melolontha larvae are susceptible to several pathogens indigenous to the area in which these insects occur in Turkey. We isolated and identified seven bacterial strains from M. melolontha and evaluated their pathogenic activity during three hazelnut seasons from 2002 to 2004 on larvae of M. melolontha. Using various morphological, physiological, and biochemical characteristics in detail, bacterial isolates were identified as Pseudomonas sp., Bacillus thuringiensis, Pseudomonas sp., Enterobacter sp., B. sphaericus, Acinetobacter sp., and B. weihenstephanensis. The insecticidal activity of isolates at 1.8 x 10(9) bacteria/ml dose, within 10 days on the larvae of M. melolontha are 40% for Pseudomonas sp., 80% for Bacillus thuringiensis, 50% for Pseudomonas sp., 20% for Enterobacter sp., 60% for B. sphaericus, and 80% for B. weihenstephanensis. We also purified crystals from B. thuringiensis and B. sphaericus and tested the insecticidal activity on the larvae of M. melolontha. In crystal protein bioassays, the highest insecticidal effect detected was 70% with crystals of B. thuringiensis. Our results indicate that indigenous B. thuringiensis and B. weihenstephanensis isolates and crystal of B. thuringiensis may be valuable as biological control agents.