Populations of Tetranychus urticae Koch were studied to determine their influence on photosynthesis of leaves of potted ‘Franklin’ and ‘Red Delicious’ apple trees. Nine days after treatment, 15, 30, and 60 mites/leaf had reduced net photosynthesis values 26, 30, and 43% below check Franklin leaves. Significant negative correlations were found between photosynthesis and leaf color values. On Red Delicious leaves, initial population levels of 15 and 30 mites/leaf reduced photosynthesis as early as 7 days after treatment. Significant negative correlations were found among photosynthesis, mite populations, and color values. Observers could not detect visible yellowing of leaves even when there were reductions in net photosynthesis.