Thirty-seven entomopathogenic bacteria isolated from six common pests of hazelnut in the Black Sea Region of Turkey have been screened for their potential of antibacterial substance production against indicator bacteria by the agar spot assay and well diffusion assay. Results indicated that 13.5% of entomopathogenic bacteria, Pseudomonas fluorescens (Pf-Xd1), Bacillus polymyxa (Bp-Ar2), Bacillus thuringiensis (Bt-Bn1), Serratia marcescens (Sm-Mm3) and Pseudomonas flourescens (Pf-Aa4) isolated from pests of Xyleborus dispar, Anoplus roboris, Balaninus nucum, Melolontha melolontha and Agelastica alni, respectively, showed significant levels of inhibitor activities against indicator bacteria. Well diffusion assay showed that supernatants of Bp-Ar2, Bt-Bn1 and Sm-Mm3 have antibacterial activity, whereas Pf-Xd1 and Pf-Aa4 did not show any activity. Furthermore, the antibacterial activity of the substance produced by the Bp-Ar2 has a narrow spectrum, whereas those of Bt-Bn1 and Sm-Mm3 exhibit broad spectrum. The production of these antibacterial substances were similarly determined at early logarithmic phase in the growth cycle of three bacteria and continued until the beginning of the stationary phase as primer metabolite. In addition, optimal pH (at 7-9 forBt-Bn1 and 5-9 forSm-Mm3), medium (Muller Hinton broth forBt-Bn1 and Luria Bertani broth forSm-Mm3), temperature (25A degrees C for Bt-Bn1 and Sm-Mm3) and production time (24h forBt-Bn1 and 72h forSm-Mm3) of these substances were determined. Our results demonstrate that entomopathogenic bacteria are a potential source of antibacterial substances.