none Prunus mahaleb L. in Bari province (Southern Italy) grows mainly wild, and is the most common root-stock for Prunus avium L. (sweet cherry). During three years of observations, 20-30% of its fruits were found to be infested by Rhagoletis cerasi L. Full-grown larvae of this pest on P. mahaleb are parasitized by Opius magnus Fischer, an endophagous parasite, frequently at the rate of 10-30%. No parasitization occurs when R. cerasi larvae live in sweet cherries. P. mahaleb seems to be the only alternative host of Rhagoletis because Lonicera spp. were not found to be infested. Infestations by R. cerasi on P. mahaleb occur mostly in June, or at the beginning of July, about one month later than the infestations on P. avium. The first adults of O. magnus appears towards the middle of May. The first generation lasts one month; the second starts in July and hibernates underground in the puparia of the host until May of the following year.