We studied host selection behaviour of Asobara tabida when it had the choice between different age classes of Drosophila melanogaster larvae. A. tabida attacked early second instars (24 h at 25° C) with more success than early third and late third instars (respectively 48 h and 72 h at 25° C). The third instars often escaped parasitization because the parasitoid failed to pierce their thick skin with its ovipositor, or because they frightened off the parasitoid by vigorous wriggling. The main mortality factor of A. tabida eggs in D. melanogaster larvae was encapsu- lation by blood cells of the host. The proportion of encapsulated A. tabida eggs increased with the age of the larvae at the moment of parasitization. The highest percentage of parasitism was found in early second instars, the stage that represents the best survival probability. A. tabida rejected a substantial proportion of larvae after piercing them with its ovipositor. We think that this happens because D. melanogaster, which is an alien species in western Europe, is not the preferred host species of A. tabida.