The structuring of canopy arthropod communities was reviewed and investigated in relation to tree species diversity and its component factors, interspersion of different species and density of each tree species. Fifteen treatments of Douglas-fir (Pseudotsuga menziesii) and red alder (Alnus rubra) (various densities and proportions of each) were randomly assigned to 0.073 ha plots, replicated three-fold at each of two locations in Western Oregon: the Cascade Head Experimental Forest and the H.J. Andrews Experimental Forest. The six treatments used in this study were two densities of Douglas-fir and red alder monoculture (1000 trees/ha and 500 trees/ha), and mixtures of Douglas-fir and red alder (500 trees/ha of each) planted simultaneously or red alder planted 6 years after the Douglas-fir. Trees were initially planted in 1985-1986. The arthropod communities were sampled in the summer of 1998 by bagging and pruning branches from the mid-canopy of both tree species. Multivariate analyses distinguished the arthropod communities found on each tree species and geographical location, but not among the different diversity and density treatments. Many arthropod taxa and functional groups residing on a single tree species had significantly different abundances between locations. The most commonly encountered taxon, Adelges cooleyi Gillette (Homoptera: Adelgidae), was most abundant on Douglas-firs in the 500 trees/ha monoculture and the mixture with younger red alder, and least abundant in the mixture with both species planted simultaneously (the 1000 trees/ha Douglas-fir monoculture was intermediate). Adelgids showed no significant response to location, but did respond to combinations of location x treatment. The functional group of sap-feeders was dominated by adelgids, and showed similar treatment differences on Douglas-fir. Defoliators on red alder responded in abundance to location, treatment (most abundant in the 500 trees/ha monoculture and even-aged mixture, least abundant in the 1000 trees/ha monoculture), and location x treatment. This study demonstrated that tree species and geographical location are the primary determinants of forest arthropod community composition at this spatial scale. However, tree species diversity and density can affect the abundance of certain arthropod taxa, apparently through some combination of resource quality and plant apparency. / Graduation date: 2000