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Successful application of entomopathogenic fungi as biocontrol agents requires a selection of highly virulent isolates with the ability to tolerate abiotic stress factors. Herein, we evaluated the entomopathogenic activity as well as the effect of ultraviolet‐B radiation (UV‐B) and water stress on conidia germination, viability, and fungal growth of three Fusarium isolates, namely Fusarium oxysporum NS1, F. solani OS11, and Fusarium sp. Pi21. Pathogenicity was tested against pupae and adults of the Mediterranean fruit fly, Ceratitis capitata Wiedemann (Diptera: Tephritidae), by spraying and by body‐contact bioassays, respectively. Average mortality of treated individuals ranged from 62 to 93% for pupae and from 26 to 86% for adults, depending on the fungal isolate and concentration. The highest mortality rates were caused by F. oxysporum NS1 and F. solani OS11. These isolates were effective within a short time period (LT50, pupae: <87 h, adults: <68 h) using low concentrations (LC90, pupae: <7 × 105 conidia ml−1, adults: <5 × 106 conidia ml−1). Sensitivity to UV‐B and water activity (aw) varies significantly among the studied isolates. Fusarium oxysporum NS1 was the most tolerant to UV exposure and water stress, with >63% relative conidia germination and growth rates after 6 h UV exposure and >50% conidia germination and viability at aw = 0.928. These results showed the high pathogenicity of Fusarium isolates, especially F. oxysporum NS1, against C. capitata and their tolerance to abiotic stress conditions. However, the application of these isolates in the field as biopesticides requires further studies elucidating their specificity toward C. capitata and their safety for non‐target organisms. [ABSTRACT FROM AUTHOR]