Background: Olives have been cultivated in the Mediterranean region and in Croatia for more than 2, 500 years. Croatia has extremely favourable conditions for olive cultivation, and indigenous varieties and those introduced from Italy are grown on an area of 18, 600 hectares. Olive cultivation, and thus the production of quality olive oil, is limited by pests, the most important of which is the olive fruit fly. Olive varieties differ in their susceptibility to this pest, and 60 varieties are grown in Croatia. Methods: The susceptibility of two indigenous (Oblica and Istarska bjelica) and four varieties introduced from Italy (Ascolana Tenera, Frantoio, Leccino and Pendolino) to olive fruit fly infestation was investigated at two important cultivation sites in Croatia (Vodnjan and Benkovac). Three trees of each cultivar were selected, and 100 olive fruits were collected from each tree. The fruits were first visually inspected for olive fruit fly infestation and then prepared for pest development monitoring according to a standardised protocol. Pest development was monitored weekly. Results: The olive grove in Vodnjan had a significantly higher percentage of fruit infested with olive fruit fly. Most flies developed in the fruits of the indigenous Istrian variety Istarska bjelica (145 specimens), while the second most infested variety was the Italian variety Frantoio (119 specimens). The varieties Oblica, Ascolana Tenera and Pendolino had the same number of flies (75 - 79 specimens), and the small-fruited variety Leccino had the lowest number of flies (70 specimens). In the olive grove of Benkovac, the intensity of olive fruit fly infestation was much lower, and in this area the Ascolana Tenera variety was the most frequently infested (59 specimens), followed by Leccino (47 specimens). The varieties Pendolino, Oblica and Istarska bjelica were about equally frequently infested (26 - 39 specimens), and the variety Frantoio was the least infested (12 specimens). Conclusion: Varieties with large fruits (Ascolana Tenera) and varieties with long ripening period (Istarska bjelica and Frantoio) are more susceptible to olive fruit fly infestation than other varieties. Favourable climatic conditions for olive fruit fly development in the study areas and insufficient pest control measures in Vodnjan also contributed to the heavy infestation. Cultivation of the varieties Istarska bjelica and Ascolana Tenera is threatened by this pest in the studied areas.