Technical Abstract: Ontogenetic changes in feeding behavior of armyworms, Pseudaletia unipucta (Haworth), were compared on tall fescue [Schedonorus arundinaceus (Schreb.) Dumort] cultivars with spiny-, intermediate-, or smoothedged leaf blades to clarify whether the edge spines deter caterpillars, in which case release of modified, smooth-edged forage grasses for improved livestock performance might aggravate armyworm damage to pastures. Edge biting, success initiating edge-feeding, and propensity to window- or edge-feed were observed for individual 2nd, 3rd, or 4th instars on grass leaf blades with intact margins. Early second instars bit smooth and spiny leaf edges with equal frequency but were unable to initiate edge-feeding. They fed on leaf edges after margins were cut away, but not when leaf juice was applied to intact edges, indicating the leaf margin is a barrier. Third and 4th instars mostly edge-fed on smooth leaves, but on the grasses with spiny margins they compensated for the difficulty of edge-feeding by prolonging their window-feeding. There was no developmental cost to window- feeding by 3rd instars, but 4th instars suffered reduced weight gain on spiny grass apparently because, unlike earlier instars, their mandibles are too large and not well shaped for efficient window-feeding between the parallel vascular bundles. Armyworms display behavioral plasticity in feeding mode in response to spiny- versus smooth-edged grasses. Greater use of smooth-bladed pasture grasses may result in proportionately more edge-feeding by armyworms but is unlikely to result in markedly greater pasture losses because this grass-feeding specialist so effectively exploits conventional tall fescue despite its structural characteristics.