The Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae) is one of the most important pests of potatoes and causes great losses in potato production worldwide. Chemical insecticides are primarily used to control this pest, but this has rapidly caused insecticide resistance. In this study, 24 entomopathogenic fungi were obtained from 43 soil samples in potato fields and identified by ITS gene sequencing. Nine of the isolates were identified as Beauveriabassiana (Bals.) Vuill and 15 as Metarhizium sp. All fungal isolates were first tested against the adults and larvae of CPB under laboratory conditions. The most effective isolate was determined as B. bassiana SK-8 with 86% mortality and mycosis against adults, and 100% mortality and 80% mycosis against larvae. Therefore, isolate SK-8 was further characterized by phylogenetic analysis using bloc, rpb1 and tef gene sequences and this also confirmed that the isolate SK-8 was B. bassiana. B. bassiana SK-8 was finally tested against adults and larvae of CPB under field conditions. LC50 values were estimated as 3.42 × 106 and 1.15 × 107 conidia ml–1 for adults and larvae, respectively. LC90 values were estimated as 1.12 × 109 and 4.08 × 1010 conidia ml−1 for adults and larvae, respectively. Consequently, B. bassiana SK-8 seems to be a promising biocontrol control agent against CPB.