Zetzellia mali(Ewing) feeds on and is fed upon byTyphlodromus pyriScheuten andMetaseiulus occidentalis(Nesbitt), especially when phytophagous mites are scarce on apple foliage. These mites interact indirectly through competition for prey. Probably because of both indirect and direct effects,T. pyriwas less dense andM. occidentaliswas much less dense in plots withZ. malithan in plots whereZ. maliwas suppressed by endosulfan.M. occidentalisalso had less effect onZ. malithanT. pyri. Biological control ofPanonychus ulmi(Koch) andAculus schlechtendaliNalepa was effective whereZ. malioccurred alone at high levels or with either or both phytoseiids. Plots that received mixedT. pyri–M. occidentalishad effective control of pest mites with or withoutZ. mali. Z. malidisplacedM. occidentalisand controlled pest mites in plots withM. occidentalisonly. In plots withT. pyrionly, dominance ofT. pyriorZ. maliwas mixed—most often near equal levels occurred, but sometimes one or the other dominated. In mixed plotsT. pyridisplacedM. occidentalisin 1991, but interactions betweenT. pyriandZ. maliin 1992 were like those in plots withT. pyrionly. Releases of 25–50M. occidentalisand 200–300P. ulmiinto trees with manyZ. malidid not allow for establishment ofM. occidentalis, but establishment readily occurred where fewZ. maliwere present. Management ofM. occidentalis, T. pyri, Z. mali, and pest mites is discussed in relation to long-term biological control.