Feeding damage by Pacific spider mite,Tetranychus pacificus(McGregor), or Willamette mite,Eotetranychus willametti(McGregor), was shown to reduce grape leaf photosynthesis and stomatal conductance. Negative linear regressions between feeding damage and grape leaf gas exchange were developed for both species of mites (P<0.01). Both mite species appear to have comparable effects on field photosynthetic rates and stomatal conductance of grapevines. Photosynthetic rates of undamaged adjacent leaves were not affected by mite feeding damage, whereas stomatal conductance appears to increase in undamaged adjacent leaves. In greenhouse trials, significant interaction effects were observed between cultivar type and stomatal conductance, but not between cultivar type and photosynthesis. Four insecticides did not affect grape leaf photosynthesis or stomatal conductance.