The objectives of this study were to identify the preimaginal development of instars of Diachasmimorpha tryoni (Cameron) (Hymenoptera: Braconidae: Opiinae), a larvo-pupal parasitoid of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), and to define its thermal requirements. Development was followed daily and parasitoid stage and size were noted. Larval samples were observed under a scanning electron microscope. Developmental rate as a function of temperature was studied with the Sharpe-DeMichele 6-parameter biophysical model and thermal requirements (lower developmental threshold and thermal constant) calculated using a linear regression. The duration of parasitoid development at seven temperatures ranged from 16.52 ± 0.96 days at 28 °C to 57.62 ± 1.85 days at 15 °C (males) and from 18.00 ± 0.79 days at 28 °C to 63.60 ± 1.52 days at 15 °C (females). Male development always took shorter than female development. No parasitoids emerged at 29 and 30 °C, which seems to be close to maximal temperature. The development required 322.6 ± 17.6 degree-days above a thermal threshold of 9.19 °C (both sexes cumulated). These data are compared with those of the host fruit fly, and the suitability of D. tryoni in controlling the Mediterranean fruit fly in Reunion Island is discussed.