Effects of feeding by Tetranychus urticae Koch and Eotetranychus carpini borealis (Ewing) on red raspberry physiology was investigated under greenhouse conditions. Measurements of chlorophyll fluorescence and chlorophyll contents were made from infested leaves and the most recently expanded and uninfested leaves of infested plants. Measurements were compared with those taken from similar leaves of the control plants. After 24 h of feeding, uninfested leaves of T. urticae infested plants showed lower maximum fluorescence (F_m) and higher initial fluorescence (F_o) than these of E. carpini borealis infested plants. After 2 wk, no significant difference was found for these parameters while significant increase of maximum fluorescence (F_m) and variable fluorescence (Fv) was detected for infested leaves. Half rise time (t1/2) was significantly reduced for these leaves. When compared with control, Fv/F_m, Fo, and chlorophyll contents of infested leaves were not affected by spider mite feeding following 2 wk of feeding. The results of this study indicate that cellular injury occurs before appearance of damage and that the primary site of injury for spider mite feeding may be the plastoquinone pool (QA), which plays major role in electron transport during photosynthesis. Comparisons of injury caused by the two spider mite species are made and potential implications of spider mite injury to red raspberry plants are discussed in relation to field conditions.