The alfalfa weevil,Hypera postica(Gyllenhal), is the most important leafmass consumer of alfalfa,Medicago sativaL. However, little is known about how insect defoliation injury affects photosynthetic responses of alfalfa. Studies were conducted in 1990 and 1991 to characterize the gas exchange responses of alfalfa to simulated and actual alfalfa weevil larval injury. Gas exchange responses were observed for two levels of plant organization: (1) the remaining tissue of injured leaves and (2) the remaining leaves of defoliated plants. Transpiration and stomatal conductance rates for leaves with actual and simulated alfalfa weevil injury were greater than for leaves without defoliation injury. However, photosynthetic rates of leaves with simulated and actual alfalfa weevil injury were not significantly different from each other or from leaves without defoliation injury. Consequently, both simulated and actual alfalfa weevil injury did not seem to affect the photosynthetic apparatus of the remaining leaf tissue. Therefore, the principal effect of alfalfa weevil defoliation injury on photosynthetic responses of alfalfa was to reduce the amount of photosynthesizing leaf tissue, but not photosynthetic rates. Moreover, simulated alfalfa weevil injury techniques produced photosynthetic responses that were not significantly different from actual alfalfa weevil injury. Defoliated plants responded to simulated alfalfa weevil injury by altering the normal senescence pattern of the remaining leaves. In particular, leaves of defoliated plants maintained higher photosynthetic rates than corresponding leaves of control plants, which showed normal senescence patterns of decreasing photosynthetic rates. As defoliation injury increased in magnitude, leaf senescence of the remaining leaves proceeded at a progressively slower rate. Delayed leaf senescence seems to be a plant compensatory response to alfalfa weevil defoliation injury.