Bactrocera cucurbitae and Dacus ciliates are two fruit fly species damaging cucurbit crops. Of Asiatic origin, B. cucurbitae has become an invasive pest in many countries especially on the African continent. Conversely, D. ciliates, of African origine, expanded its distribution area during the last years and is now becoming a pest in different Asiatic countries. Both species rank among invasive pests and have been introduced on La Réunion, a French island in the South-Western Indian Ocean, over the last 50 years. They now damage most cucurbits cultivated in the island and cause important yield losses. La Reunion is a subtropical island, with high altitudinal gradient (up to 3000 m) and two seasons (hot and rainy summer, milder and drier winter). Nevertheless, the two species are found in most areas, regardless of season and altitude. However, they show some altitudinal preferences: B. cucurbitae is more abundant in the lowlands and D. ciliates at medium altitudes. The aim of this work was to characterize the population structure of B. cucurbitae and D. ciliates, test host races and seasonal or altitude effects.