Effects of host density and plant species diversity on populations of Colorado potato beetle,Leptinotarsa decemlineata(Say), were studied in experimental field plots. Hosts were grown at two densities and four levels of plant diversity: potato monoculture, potato/bean biculture, potato/bean/weed triculture, and potatoes grown in fallow plots. All life stages of the beetle were affected by both host density and plant diversity. Numbers of beetles per plant were highest in low-diversity cultures of low host density; higher host density or plant diversity was accompanied by a reduction in numbers of beetles per plant. Differences in host-plant size among treatments were partially responsible for these trends. When beetle densities were expressed as numbers per unit leaf area, treatment effects were lessened or lost. Changes in beetle density among treatments were accompanied by differences in fecundity among treatments, but not by differences in survival. Fecundity, calculated as numbers of eggs deposited per adult, was lower in high-diversity than in low-diversity plots. Survival rates (egg to late instar) and predation on eggs were not affected by plant species diversity.