Under field conditions, densities of 0, 4, or 8 adult female potato leafhoppers,Empoasca fabae(Harris), were caged on individual alfalfa plants during the second and second-third crop growth cycles. Plants were destructively sampled three times in each crop growth cycle. Injury byE. fabaecaused the percentage of total nonstructural carbohydrates to be much higher in leaves of infested plants. Leaf photosynthesis of plants caged with 4 or 8E. fabaewas reduced by 60 and 80%, respectively. The effects of feeding byE. fabaeon leaf transpiration were very similar to that shown for photosynthesis. Stomatal resistance increased from11 s cm−1on infested plants. Leaf crude protein of plants infested with either 4 or 8E. fabaewas reduced by 22–35% after only 1 wk of leafhopper feeding.