SummaryThe olive fruit flyBactrocera oleae(Rossi) (Diptera: Tephritidae) is the major pest of olives worldwide. The figitid wasp,Aganaspis daci(Hymenoptera: Figitidae), is a larval-prepupal endoparasitoid of fruit fly species, and it was found to successfully parasitize medfly larvae in field-infested figs in Greece. To assess the potential ofA. dacias a biological control agent againstB. oleae, we studied the effect of olive fruit size on parasitism rates ofA. dacion 2ndand 3rdinstar larvae ofB. oleae, by using fruit of different size (cultivar ‘Chalkidikis’) and wild olive fruit. In addition, we conducted releases ofA. dacifemales in a pilot olive grove in Volos, Magnesia. From July to October, we released 200A. dacifemales/0.1 ha/week, followed by olive fruit sampling to estimate olive fruit infestation levels and the parasitism rates ofA. daci. Laboratory trials revealed that fruit size and larvae instar were predictors of parasitism success ofA. daci, with parasitism rates higher for small-size fruit of the cultivar “Chalkidikis” and the 3rdinstar larvae ofB. oleae. In field trials, noA. daciadults emerged from the olive fly infested fruit.