The alder leaf beetle (Agelastica alni L., Coleoptera: Chrysomelidae) is one of the sources of damage to hazelnut and alder trees throughout the world. Chemical substances have been utilized to control this pest, In the present study, in order to find a more effective and safe biological control agent against A. alni, we investigated the bacterial flora of the alder leaf beetles that were collected from the vicinity of Trabzon, Turkey, during 2000-2002, and tested them for insecticidal activities on it. It was determined that the number of total bacteria is 9.8 x 10(4) +/- 0.121 bacteria/larva and 1.31 X 10(5) +/- 0.310 bacteria/adult. Based on morphological, physiological and biochemical tests, bacterial flora were identified as Enterobacter agglomerans (Aa1), Listeria sp. (Aa2), Pseudomonas chlororaphis (Aa3) and Pseudomonas fluorescens (Aa4). The highest insecticidal effect determined on both the larvae and adults of A. alni were 70% and 56% with Aa4 within seven days, respectively. The insecticidal activities of the other isolates (Aa1, Aa2 and Aa3) were determined as 65%, 48% and 37% on the larvae, and 50%, 34% and 30% on the adults of A. alni, respectively.