Laboratory tests were conducted to evaluate the possible effect of a deliberately introduced fruit fly parasitoid, Diachasmimorpha tryoni, on 2 non-target flowerhead-feeding tephritid flies, Trupanea dubautiae and Ensina sonchi. The former is an endemic Hawaiian tephritid which feeds on flowerheads of the native composite shrub, Dubautia raillardioides; the latter is an inadvertently introduced tephritid infesting flowerheads of the exotic weed, Sonchus oleraceus. Gravid females of D. tryoni were confined in test cages with field-collected D. raillardioides and S. oleraceus flowerheads infested with late instars of T. dubautiae and E. sonchi, respectively. D. tryoni showed low levels of visiting and probing responses to D. raillardioides flowerheads and relatively higher responses to S. oleraceus flowerheads in both the presence and absence of the parasitoid's normal host, the Mediterranean fruit fly, Ceratitis capitata. With 72-h exposure to D. tryoni, 13.9 and 2.6% of T. dubautiae were attacked by test parasitoids in the absence and presence of C. capitata in the test cage, respectively; while 56.8 and 34.2% of E. sonchi were attacked. In contrast, 94.0 and 84.0% of C. capitata larvae presented in screened dishes with diet in test cages were attacked by D. tryoni in tests with T. dubautiae and E. sonchi, respectively. Attack of D. tryoni on flowerhead-feeding T. dubautiae and E. sonchi resulted in significant reduction in the emergence of adult flies, especially in the absence of the parasitoid's normal host. While 8.8-12.8 adult D. tryoni per test (both males and females) successfully emerged from medflies, no adult D. tryoni progeny emerged from T. dubautiae, and only 3 deformed males of D. tryoni (much smaller than the normal wasp) emerged from E. sonchi. The relevance of these findings to the safety of future biological control programs against tephritid pests is discussed. none