Studies examined the possibility that mixtures of endophytic perennial ryegrass and Kentucky bluegrass provide resistance against larvae of the black cutworm, Agrotis ipsilon (Hufnagel). We hypothesized that resistance against A. ipsilon in such stands would stem from the influence of Kentucky bluegrass on A. ipsilon growth and behavior rather than the influence of endophytic perennial ryegrass. In replicated greenhouse experiments, black cutworm larvae initially emigrated more quickly from pots containing monocultures of endophytic perennial ryegrass than from Kentucky bluegrass monocultures or polycultures of Kentucky bluegrass and endophytic perennial ryegrass. However, biomass of emigrating larvae decreased linearly as the proportion of Kentucky bluegrass increased. Turfgrass mixtures containing endophytic perennial ryegrass and Kentucky bluegrass may provide resistance against A. ipsilon mainly through the physiological effects of Kentucky bluegrass on A. ipsilon growth and development, but possibly through the influence of endophytic perennial ryegrass on A. ipsilon movement and foraging behavior as well.