Abstract: Isaria fumosorosea (syn. Paecilomyces fumosoroseus) (WIZE) Brown & Smith(Deuteromycota) is potentially useful for the biological control of economically importantagricultural and forest insect pests. Selection of effective, highly virulent strains is a prerequisitefor development of successful biological control agents. Our strain CCM 8367 of I. fumosoroseawas isolated from a lepidopteran pest. The aim of our study was to evaluate the effects of thisstrain on larvae and pupae of the Colorado potato beetle, Leptinotarsa decemlineata (Say)(Coleoptera: Chrysomelidae). In addition, we compared it with the effects of I. fumosoroseastrain isolated from the commercial product PreFeRal® WG (Biobest, Belgium; I. fumosoroseastrain Apopka 97 as an active ingredient). Obtained results revealed high virulence of CCM 8367blastospores to the late last instar larvae, prepupae and pupae of L. decemlineata. Obviousdifferences in virulence between our strain and the strain isolated from PreFeRal® WG werefound using fresh blastospores, where corrected mortality of CCM 8367-treated larvae eight daysafter the treatment was 96.5% while mortality in Apopka 97-treated larvae reached only 54.4%.We can conclude that the strain I. fumosorosea CCM 8367 has strong insecticidal effects onL. decemlineata and has a potential to be implemented as a novel biocontrol agent.