Aggregation and species association of 9 species of phytophagous and predaceous mites were studied for 6 yr in an orchard with 100, 2 m tall 'Red Delicious' apple trees. To quantify aggregation, a method was developed that is powerful, and easily interpreted. It relates the proportion of habitat occupied to mean density of the organism, and allows discrimination of aggregation differences between data sets. We found that plant feeders were more aggregated than predators, and active life stages were less aggregated than eggs. Specifically, webspinning spider mites (Tetranychus urticae + Eotetranychus sp.) were the most aggregated. Panonychus ulmi was less aggregated than the webspinning mites, and Bryobia rubrioculus was the least aggregated of the spider mites. Zetzellia mali, the slowest moving predator, had the greatest aggregation of all the predators. The rapid moving phytoseiids were the least aggregated of all the mite species studied, with the specialist predator Metaseiulus occidentalis having the greatest aggregation, the generalist predator Typhlodromus pyri having medium aggregation, and the fast moving Amblyseius andersoni having the least aggregation. Predator-predator, predator-prey, and prey-prey associations were measured using Yule's V association index. Predator-predator associations were the strongest and most consistent, showing a consistent seasonal pattern of neutral-negativeneutral association. Negative associations of T. pyri with other predators were the strongest, which is consistent with evidence that this mite can detect other predators. Predator-prey seasonal associations were weak and mixed, and interactions between prey species were generally weakly positive, probably because of similar habitat preferences. Predaceous mites were generally more aggregated when competing with other predators, possibly allowing the coexistence of 3 predators simultaneously for 6 years via mechanisms proposed by the "aggregation theory of coexistence". G. occidentalis showed the greatest change of aggregation when other predators were present, Z. mali and T. pyri also showed significant changes in aggregation when they were with other predators, but A. andersoni (the largest, fastest predator in our study) showed no changes in aggregation. T. pyri's aggregation increased the most when in the presence of Z. mali, perhaps because of egg predation by the stigmaeid, or because T. pyri could detect the other predator. / Graduation date: 1999