Effects of plant-host type, either wheat,Triticum aestivumL., or oat,Avena sativaL., and the fungal entomopathogenBeauveria bassiana(Balsamo) Vuillemin, on numerical and spatial distributions of Russian wheat aphid,Diuraphis noxiaKurdyumov, populations were investigated. Untreated adult aphids or aphids inoculated with conidial suspensions of the pathogen were released onto the center seedlings of an Bl-seedling grid in a cage. After 17 d, the numbers of live adults, live nymphs, and sporulating cadavers were recorded for each plant, along with plant height and dry weight. Aphid populations were consistently lower on oat than on wheat, and lower inB. bassianatreatments. Spatial statistics (geostatistics) were used to compare patterns of aphid distribution on plants. On wheat without the pathogen present, adult aphid populations were highest in approximately concentric rings near the outermost plants in grids. Addition ofB. bassianato the system reduced the amount of variation caused by spatial orientation, thereby changing the spatial distribution of aphid populations. On oat, aphid populations generally were highest near the center of the grid, but addition of the pathogen changed the spatial structure of the aphid population. Results suggest that aphid movement behavior, which is intimately linked to host-plant quality, is an important determinant of epizootic proliferation. Elucidation of these relationships may eventually lead to a synergistic use of microbial control agents and host-plant resistance in pest management strategies.