Beauveria bassiana (Balsamo) Vuillemin and Metarhizium anisopliae var. anisopliae (Metschnikoff) Sorokin are the most common entomopathogenic fungi used in microbial control programs all over the world. Assessments of the genetic variability of these 2 important species are useful for the development of effective biocontrol strategies and for evaluating the impact of artificial epizootics. In this study, the genetic diversity of 13 B. bassiana and 33 M. anisopliae var. anisopliae strains isolated from the hazelnut-growing region of Turkey was determined by using the amplified fragment length polymorphism (AFLP) and alpha- and beta-isoenzyme analyses. Cluster analysis of AFLP data clearly separated both B. bassiana and M. anisopliae var. anisopliae strains into 3 and 4 different groups, respectively. While alpha- and beta-esterase banding patterns clearly separated M. anisopliae var. anisopliae strains, it did not give enough information about B. bassiana strains. We also assessed the growing ability of all isolates at different temperatures (8, 16, 25, and 37 degrees C) and UV exposures (30 and 60 min), and virulence against Tenebrio molitor. These results indicated that there is a significant variability within the B. bassiana and M. anisopliae var. anisopliae populations in this region. Although the diversity of B. bassiana isolates is associated with geographic location, it is not associated with habitat type. There is also no association amongst B. bassiana isolates in terms of the ability to grow at different temperatures and UV exposures, or virulence against T molitor. The diversity of M. anisopliae var. anisopliae strains is neither associated with habitat type nor geographic location, and there is no association amongst M. anisopliae var. anisopliae strains in terms of the ability to grow at different temperatures (except for 16 degrees C) and UV exposure, or virulence against T. molitor. The data presented here might be useful for controlling some hazelnut pests in this region.