Abstract: The effect of temperature on the mycelial growth of 28 isolates of theentomopathogenic fungus Beauveria bassiana (Vuill.) Balsam (Ascomycota, Hypocreales,Cordycipitaceae) and five of Metarhizium anisopliae (Metch.) Sorok. (Ascomycota, Hypocreales,Clavicipitaceae) was evaluated in vitro on Sabouraud Dextrose Agar with yeast extract (SDAY).The five temperatures 15, 20, 25, 30 and 35°C were tested, and the fungal radial growth wasmeasured over 15 days at two-day intervals. Radius data were used to calculate the growth rate(mm d-1) per each temperature, isolate and replicate by performing linear regressions, and growthrate data were used for an analysis of covariance. Optimal and maximum temperatures andmaximum growth rate were estimated by fitting a modified β function to the growth rate data.The thermal characterization of entomopathogenic fungal candidates for microbiological pestcontrol is essential to assess their applicability in the field. According to the Bio – Rationalapproach, virulence against the target insect should be considered together with the adaptabilityto the thermal condition in the habitat involved. In particular, tolerance to high temperatures is aprerequisite for the application of entomopathogenic fungi in microbiological pest control in theMediterranean areas.