none Tests were done in the laboratory to determine the toxicity of the organosilicone surfactant Silwet L-77 against three tephritid fruit flies, Ceratitis capitata (Weidemann), Bactrocera dorsalis (Hendel), and B. cucurbitae (Coquillett). The biological activity of Silwet was compared with diazinon by exposing C. capitata puparia to sand treated with each chemical within 1 h, 7 d, and 14 d after treatment. Tests were also done to determine the effect of Silwet against the endoparasitoids Diachasmimorpha longicaudata (Ashmead), Tetrastichus giffardianus (Silvestri), and Psyttalia fletcheri (Silvestri). Silwet killed C. capitata and B. dorsalis at concentrations within the range of field application rates. Response of B. cucurbitae was the most variable and very high concentrations were required to kill >90% of puparia. A 75% decrease in mortality of C. capitata puparia occurred 14 d after sand treatment with diazinon. In contrast, Silwet remained as toxic to C. capitata puparia at 14 d as on the 1st d of treatment. All 3 parasitoid species were susceptible to diazinon and Silwet when greater than or equal to 0.001 and greater than or equal to 0.05% (AI) respectively, (wt:vol in water) were added to sand. Because Silwet has very low mammalian toxicity, it is of considerable interest for larger scale testing for fruit flies. However, more work is needed at the field level to determine possible adverse effects to nontarget species.