Photosynthetic rates and stomatal conductances of cotton, Gossypium hirsutum L., treated at different action thresholds for whiteflies, Bemisia argentifolii Bellows and Perring, were measured on 12 d from July to August 1995 in the Imperial Valley, California, using a LI-6200 portable photosynthesis system. Action thresholds were 2.5, 5, 10, and 20 adults per leaf. Untreated plots were also used for comparison in a 5 x 5 Latin Square Design. Photosynthetic rates of untreated cotton leaves were significantly lower than those of 2.5, 5, and 10 threshold treatment leaves on at least 9 of the 12 d. Rate differences between 2.5, 5, and 10 threshold treatments were generally nonsignificant. Photosynthetic rates of leaves treated at 20 adults per leaf were lower than those of other treated leaves on 2 or 3 d and were significantly higher than those on untreated leaves on 3 d. In general, no significant differences were detected between stomatal conductances from the 2.5, 5, and 10 threshold treatment leaves, and results were similar to the pattern seen with photosynthesis. Adult, egg, and nymphal densities on untreated cotton were usually significantly higher than those on all other treatments, but in general no differences in densities of any stage were seen in 2.5, 5, and 10 threshold treatments. Densities from the 20 threshold treatment were generally not significantly different from untreated nor from 2.5, 5, and 10 threshold treatments. Based on photosynthetic rates, stomatal conductances, and immature whitefly densities, the best action threshold on cotton in the Imperial Valley seems to be between 10 and 20 adult whiteflies per leaf.