Studies were conducted to examine the physiological response of peppermint,Mentha piperitaL., to leaf water stress induced by feeding of the twospotted spider mite,Tetranychus urticaeKoch (Acari: Tetranychidae). Levels of mite injury were estimated by a leaf injury index developed for these studies. Symptoms of water stress in mite-injured leaves were: a significant reduction in leaf fresh weight; reduced leaf specific weight (mg of fresh weight per cm2of leaf area); and accumulation of soluble leaf carbohydrates. Soluble leaf carbohydrates of peppermint are sucrose (ca. 50%) and the galactosyl oligosaccharides raffinose and stachyose. Leaf starch content was unaffected by injury at all injury levels. Accumulation of soluble leaf carbohydrate in water-stressed leaves may help mite-injured leaves maintain cell turgor pressure through an osmotic adjustment (osmoregulation) mechanism. A new method for in situ starch hydrolysis for quantitative analysis of leaf starch also is presented.