The effect of malathion-bait sprays, directed against Mediterranean fruit fly (Ceratitis capitata [Wiedemann]), on biological control of selected non-target insects was investigated in northern California during 1982 and 1983. In Stanislaus County, 19 applications of malathion-bait spray were applied over a period of 7 months; following cessation of the spray program, increases of olive scale (Parlatoria oleae [Colvee]) and black scale (Saisettia oleae [Olivier]) on olive and of brown soft scale (Coccus hesperidium Linn.) and black scale on citrus were detected, compared to population levels of these scales in the adjacent unsprayed zone. The secondary outbreaks of these scales were attributed to destruction of natural enemies (chiefly parasites) by malathion. The outbreaks of black scale (citrus and olive) and brown soft scale were not apparent until 1 year after the first application of malathion-bait spray, or 5 months after the last application. Populations of cottony-cushion scale (Icerya purchasi Maskell), citrophilus mealybug (Pseudococcus calceolariae [Maskell]) and citricola scale (Coccus pseudomagnoliarum [Kuwana]) remained at low levels on citrus in the spray zone; no treatment effect on these species was detected. Latania scale (Hemeberlesia lataniae [Signoret]) on olive was apparently suppressed by the sprays. Walnut aphid (Chromaphis juglandicola [Kaltenbach]) in Commercial walnut orchards was less abundant in the spray zone 2 months after the last application of malathion-bait; no suitable explanation for this was available. In the city of Stockton, a secondary outbreak of walnut aphid occurred in the spray zone (during the spray period) whereas an outbreak of black scale on olive was detected shortly after the spray program ended. These outbreaks were attributed to destruction of natural enemies by malathion. In general, concentrations of malathion-bait sufficient to kill most adult parasites tested were less toxic to the pest species tested. These results indicate that future (medfly) eradication programs which employ numerous sequential applications of malathion-bait spray can be expected to disrupt a substantial portion of the biological control which exists in the target zone.