RAPD-PCR was used for the molecular characterization of 58 isolates of the entomopathogenic fungus Beauveria brongniartii obtained from the European cockchafer (Melolontha melolontha) in Valle d'Aosta, Italy. RAPD band patterns were compared to site of isolation and virulence against M. melolontha larvae. Results showed genetic heterogeneity in the natural population of the fungus, and identified within this two geographically restricted groups that may represent colonization by a single isolate. No correlation between pathogenicity and grouping according to RAPD patterns was found. The findings indicate that RAPD derived markers can be used to differentiate and identify isolates within this specialized group of pathogens, and provide a rapid method for tracking new introductions in the environment.