The main objective of this study was to determine the extent to which host acceptance behaviour as related to host species, age, and defensive behaviour might explain the differences in host use that exist between two congeneric and sympatric species of parasitic wasps. Cotesia glomerata (L.) (Hymenoptera: Braconidae) is gregarious and generalist on several species of Pieridae, whereas C. rubecula (Marshall) is solitary and specific to Pieris rapae (L.). Cotesia species differed in their responses to host species (P. brassicae (L.), P. napi (L.) and P. rapae) and developmental stage (early and late 1st, 2nd and 3rd instars). In no-choice tests, host acceptance by C. rubecula was higher for P. rapae and females did not distinguish among the 6 host ages. In contrast, when foraging for P. brassicae and P. napi, C. rubecula females more readily attacked early first instar. Cotesia glomerata showed a higher degree of behavioural plasticity towards acceptance of Pieris host species and host age than did C. rubecula. Cotesia glomerata females parasitized the three Pieris species and showed higher acceptance of first and second instars over third instar. Oviposition success was also influenced by host defensive behaviour. The frequency and the effectiveness of defensive behaviour rose with increasing age of the host, P. brassicae being the most aggressive Pieris species. Furthermore, the mean duration of C. glomerata oviposition was significantly reduced by the defensive reactions of P. brassicae, which would likely affect parasitoid fitness as oviposition time is positively correlated to clutch size in C. glomerata. Acceptance frequencies corresponded well to field reports of Pieris-Cotesia associations and to patterns of parasitoid larval performance, suggesting that the acceptance phase might be used as a reliable indicator of Cotesia host-specificity.