The bronze bug, Thaumastocoris peregrinus (Heteroptera: Thaumastocoridae) is a small sap feeding hemipteran bug that has becomea serious pest of Eucalyptus plantations worldwide. Affected trees display foliage blight and defoliation, thus reducing photosynthesis and sometimes leading to the death of those highly infested mature trees. The use of entomopathogenic fungi as biological control agents is seen as an environmentally friendly alternative for pest control being chemical control seldom used in commercial forestry because its environmental and economical disadvantages. In this study, naturally occurring entomopathogenic fungi on T. peregrinus were isolated from dead and mycosed insects found in Eucalyptus plantations in Uruguay. Fungal species were identified based on spore and reproductive structures, morphology, culture characteristics, and molecular identification by amplification of the ribosomal DNA comprising ITS1, ITS2 and the 5.8S subunit. The identified entomopathogenic species belong to the genera Beauveria, Isaria, Lecanicillium, Purpureocillium and Pochonia. Bioassays using fiftyeight entomopathogenic fungal isolates from bronze bug and other forestry and agricultural pest insects were performed to determine their pathogenicity and virulence against T. peregrinus. A first in vitro screening was made by spraying 10 7 conidia/ml suspensions onto adults of T. peregrinus reared in mesh cages on detached leaf- bearing twigs of E. tereticornis. Isolates were classified into four categories based on the number of days to reach 90% of mortality. Most of the tested strains (80%) were pathogenic to bronze bug showing different degrees of virulence. For a subset of strains, values of LC 50 and LT50 were determined. In addition, the most promising strains are being subjected to mass production studies in order to identify cost-effective production methods and to select the most stable and virulent formulated strains.