Predation experiments were conducted in the laboratory to study the potential effect ofOrius tristicolor(White) on the widely used spider mite predator,Phytoseiulus persimilisAthias-Henriot (Acarina: Phytoseiidae). In a 24-h test on English cucumber leaf disks surrounded by a water barrier, we measured rates of escape and mortality ofP. persimiliswhen exposed toO. tristicoloralone or in the presence of western flower thrips,Frankliniella occidentalis(Pergande) (Thysanoptera: Thripidae), and two-spotted spider mites,Tetranychus urticaeKoch (Acarina: Tetranychidae), which acted as alternate prey.O. tristicolorkilledP. persimilisin significant numbers even when thrips were available. The addition of spider mites and thrips to the two predator system significantly reducedO. tristicolorpredation on P. persimilis and thrips. Spider mites were shown to reduceO. tristicolorpredation both through adult mites substituting as acceptable prey and through the mite's webbing acting to lower overall preying activity ofO. tristicolor. When spider mites were available, a negative correlation was observed between thrips killed andP. persimilissurviving, indicating no clear preference ofO. tristicolorfor attacking thrips rather thanP. persimilis. We discuss the significance of the generalized character ofO. tristicolorattack behavior with respect to its use as a thrips predator in the greenhouse, and the potential significance of our laboratory observations for situations requiring biological control of multiple pests.