In a 2-yr study, population densities of and damage by second-generation European corn borer,Ostrinia nubilalis(Hübner), larvae were compared among corn plantings containing or lacking different weed communities. Treatments were corn without weeds, corn principally with broadleaves, corn principally with grasses, and corn with a mixture of broadleaves and grasses. In both years, larval density (number of larvae per corn plant) and damage (number of tunnels per corn plant) were significantly lower in treatments with broadleaves than in treatments without broadleaves. Larval density was significantly lower in treatments with grasses than in treatments without grasses in 1989. The broadleaf × grass interaction was significant for larval density in 1989, and significant for damage in both years. Although natural enemies ofO. nubilalismay be more abundant and effective in weedy corn, the presence of weeds, particularly grasses, attracts adult moths to cornfields, which may result in larger infestations of corn and greater damage. In addition, weeds may harbor other pests of corn and compete with corn for nutrients and water, leading to reduced yields.