With the aim to broaden the natural enemies complex available for the biological control of key tephritid fruit flies (Diptera) of the Mediterranean Basin, the South-East Asiatic oo-pupal parasitoid Fopius arisanus (Sonan) (Hymenoptera: Braconidae) was imported in Italy in 1999. Laboratory studies allowed to develop effective low cost rearing techniques of this parasitoid on the secondary host Ceratitis capitata (Wiedemann) (Diptera: Tephritidae) using artificial devices. Moreover, we ascertained the suitability of a new host to the parasitization by F. arisanus: the olive fruit fly Bactrocera oleae (Gmelin) (Diptera: Tephritidae), key pest of olive groves in all the Mediterranean region. Preliminary field cage tests were carried out in 2000 to evaluate the survival and the parasitization ability in a typical olive area of central Italy. Results evidenced better performances of the parasitoid during autumn, rather than in summer. The parasitoid showed the shortest longevi! ty in early summer (in average, 2d in July August and 8-10d in the period September-November). In 2001, and 2004 investigations were carried out in Central Italy to verify the capability of F. arisanus to mate and increase its population in open field. In 2001, about 10000 F. arisanus specimens were released in a 1ha olive grove in July, while 2000 specimens were released in a 0.5ha olive area in September 2004. In 2004, quality control tests have been also performed to ascertain the preservation of the performances in parasitizing B. oleae in field of the laboratory reared F. arisanus population. Finally, results from intrinsic competition with the autochthonous parasitoid E. urozonus were also evaluated to test the effects from the release of the exotic parasitoid on local B. oleae antagonists. The periodical recovery of olives during field tests allowed us to verify that the parasitoid was able to reproduce and gradually increase its population in field: in particular, w! e estimated parasitoid population to increase in field from 51.7 to 96 .3 emerged specimens per 1000 olives in average from late September to late October 2004. Percent parasitism increased as well: the average level was in fact about 20% in September and 25% in October, with highest daily levels of 68 and 71% respectively. F. arisanus also maintained a sex ratio not significantly different from that obtained in laboratory (F/M=0,85). Quality control tests evidenced that the rearing conditions did not seem to affect F. arisanus ability in searching and parasitizing B. oleae in field. Furthermore, E. urozonus was consistently superior than F. arisanus in intrinsic competition (only 4% of the multiparasitized hosts emerged as the latter). Both field cage and open field tests have pointed out that hot-dry climatic conditions limit the activity of F. arisanus, while under higher humidity levels, normally recorded from early September, the parasitoid show a remarkable efficiency in parasitizing B. oleae being also able to establish n field until the! end of the olive cropping season. BR Animals and man. Control. Parasites diseases and disorders. Parasites. Insect parasites. Hosts. Insect hosts. Habitat. Man made habitat. Land zones. Palaearctic region. Eurasia. Europe. Bactrocera oleae (Tephritidae): Host; Fopius arisanus (Braconidae): Parasite. Bactrocera oleae: Biological control, Hymenopteran parasites, Fopius arisanus, Cultivated land habitat, Olive groves, Italy; Fopius arisanus: Biological control, Dipteran hosts, Bactrocera oleae, Cultivated land habitat, Olive groves, Italy. ST Animalia, Arthropoda, Insecta, Diptera, Brachycera, Cyclorrhapha, Tephritidae; Animalia, Arthropoda, Insecta, Hymenoptera, Apocrita, Terebrantia, Ichneumonoidea, Braconidae. Arthropods; Hymenopterans; Insects; Invertebrates; True Flies. none