Abstract. Some insecticides have the potential to cause varying levels of phytotoxicity. This study examined 1) the time courses of photosynthetic injury in cotton (Gossypium hirsutisms L.) leaves treated with methomyl [S-methyl-N-[(methyl carbamoyl)oxy]-thioacetimidate] and 2) the relationships between carbon exchange rate (CER), stomatal conductance, the chlorophyll fluorescence parameter FX/FP, and ribulose-1,5-bisphosphate carboxylase/ oxygenase (rubisco) activity. Plots were sprayed with either 0 or 0.84 kg methomyl ha-1 when cotton was in mid-reproductive growth. Starting on the day of spraying, CER, stomatal conductance, FX/FP, and rubisco activity were measured daily for five consecutive days [4, 28, 52, 76 and 100 hours after spraying (HAS)]. In methomyl-treated leaves, CER decreased within hours after spraying, reached their lowest point at 28 HAS in experiment I and 76 HAS in experiment II, then recovered near-control levels by 100 HAS. At their lowest points, CER of methomyl-treated leaves decreased from 20 to 50% compared to controls. Stomatal conductance, FX/FP, and rubisco activity followed similar patterns to CER. Stomatal conductance was more closely related to CER than were FX/FP and rubisco activity. Chlorophyll fluorescence recovered more quickly than did CER. Rubisco activity did not decrease till after CER. From the parameters measured in this study, stomatal conductance appeared to be the major factor influencing methomyl-induced changes in CER, although all three parameters may be involved in the process of CER change.