The aim of this investigation was to determine, purify, and characterize an antibacterial substance produced by Serratia marcescens Mm3 (Sm-Mm3) isolated from a common pest of hazelnut in Turkey, Melolontha melolontha, which is also a significant pest all over the world. A highly active antibacterial substance produced by Sm-Mm3 was purified by ammonium sulfate precipitation and high-performance liquid chromatography, respectively, and identified by spectrometric analysis (TOF-MS). Finally, we showed that this molecule is heat-stable, stable in the pH range of 5-9, and insensitive to organic solvents and proteinase K. The molecular mass was determined as 479 Da. Plasmid elimination demonstrated that the gene responsible for this antibacterial substance of the Sm-Mm3 strain is located on the plasmid. Based on the determined characteristics, the antibacterial substance was identified as a bacteriocin-like substance. Although this active substance is similar to bacteriocin because of many features such as mode of action and physicochemical properties, it differs by having a low molecular weight and by being insensitive to proteinase K. This active substance has potential to control many different pathogenic bacteria in various areas, particularly in agriculture, against plant pathogenic bacteria.