Three densities of European red mite (ERM) (<i>Panonychus ulmi</i> Koch) and twospotted spider mite (TSM) (<i>Tetranychus urticae</i> (Koch)) were maintained on greenhouse-grown 'Imperial Delicious' apple (<i>Malus domestica</i>) and 'Redhaven' peach (<i>Prunus persica</i>) leaves. As ERM- and TSM-days increased, apple leaf net photosynthesis (Pn), transpiration (Tr) , and chlorophyll content (TCHL) decreased linearly. TSM was more damaging to apple leaf gas exchange than ERM. Apple water use efficiency (WUE) declined similarly with increasing mite-days for both mite species. Apple specific leaf weight (SLW) decreased with ERM-days and increased with TSM-days. Peach Pn, Tr, and TCHL declined linearly for both ERM and TSM. Both mite species were equally damaging to peach leaf gas exchange. Peach WUE decreased with increasing ERM and TSM-days. There was no mite effect on peach SLW. The results from separate apple and peach studies indicate that peach may be more tolerant to mite feeding than apple. / Master of Science