Effects of feeding injury by the twospotted spider mite,Tetranychus urticaeKoch (Acari: Tetranychidae), on 14-CO2assimilation (net photosynthesis), leaf conductance to gas exchange, and leaf chlorophyll in peppermint,Mentha piperitaL., were investigated. Rates of 14-CO2, assimilation were reduced in proportion to degree of sustained injury and probably were the result of reduced leaf conductance to gas exchange, which inhibited influx of carbon dioxide to chloroplasts. Assimilation rates for both uninjured and variously injured leaves were correlated with leaf conductance. Inhibition of gas exchange in injured leaves was the predominant factor responsible for reduced photosynthesis. Extractable leaf chlorophyll was reduced in proportion to the degree of injury sustained. Leaf injury was estimated by using an injury index developed for these studies