Abstract: Fungal entomopathogens are important antagonists of arthropod pests and have attracted increased attention as biocontrol agents in integrated pest management programs. In addition to colonizing arthropods, evidence has accumulated that some entomopathogenic fungi like Beauveria bassiana (Bals.) Vuill. (Ascomycota: Hypocreales) can endophytically colonize a wide array of plant species. For a couple of crop plants it has been proved that endophytic B. bassiana can provide a systemic protection against damage by various insect pests or might trigger induced systemic resistance mechanisms against plant pathogens. Grapevine, Vitis vinifera (L.) is regarded as an extremely input-intensive culture in particular in terms of pesticide application intensities and frequencies. Accordingly, endophytic establishment of an entomopathogenic fungus in grapevine plants would represent an alternative and sustainable plant protection strategy, with the potential of reducing pesticide applications in viticulture. Here, we report for the first time on an endophytic establishment of the entomopathogenic fungus B. bassiana in grapevine plants. We used two different commercialized B. bassiana strains and applied them as conidial suspensions or as the formulated product on potted grapevine plants. Verification of endophytic establishment of the respective B. bassiana strain was achieved by the amplification of strain-specific microsatellite markers. Survival inside leaf tissues was evident at least 21 days after inoculation, irrespective of the inoculum used. No negative effects of endophytic B. bassiana on growth of potted grapevine plants were evident. The antagonistic activity of endophytic B. bassiana against vine mealybug larvae Planococcus ficus was assessed using surface sterilized leaves in a bioassay. Mortality of first instar mealybug larvae was significantly higher during the first week of settlement on endophytic grapevine leaves compared to leaves from control plants and larvae feeding on endophytic leaves were significantly smaller than those who had fed on control leaves. Overall, endophytic establishment of an entomopathogenic fungus such as B. bassiana in grapevine plants represents an alternative and sustainable plant protection strategy, with the potential of reducing pesticide applications in viticulture.