Abstract: The Colorado potato beetle, Leptinotarsa decemlineata, is a serious insect pest, attacking mainly potato. This pest causes severe yield loss all over the world and it is difficult to control by chemical pesticides because it quickly develops resistance to them. In our study we investigated the potential of the fungus Beauveria bassiana, a natural pathogen of insects, to kill L. decemlineata. Twelve strains were isolated from cadavers of Colorado potato beetles collected in potato fields in the Czech Republic. In laboratory trials, adult beetles were treated by suspensions of conidia at the concentration of 1 × 107 spores per milliliter and their survival was recorded daily for three weeks. The results of the bioassays revealed that all new native strains were pathogenic to L. decemlineata adults and caused mortality up to 100 % at the end of the trial period. These strains were more virulent than a reference strain GHA.The selected strain of Beauveria bassiana (Bb8) was further tested in pots with potato plants and in the field and effect on the number of emerging adults was evaluated. In pot experiments, soil application of fungus B. bassiana and application on leaves significantly decreased numbers of emerging CPB adults. Mortality was almost 100 %. The field application of fungus B. bassiana significantly decreased the number of emerging CPB adults in comparison to control sites by ca. 30 %. Our findings demonstrate that most of the native B. bassiana strains especially strain Bb8 showed high virulence against this pest and suggest that this entomopathogenic fungus could be an alternate solution to broad-spectrum chemical insecticides.