Methomyl (S-methyl-N-[methylarbamoyl)oxy]-thiocetimidate) has various phytotoxic effects in cotton (Gossypium hirsutum L.). Among these effects were performed at the Texas A&M Research Farm in Burleson County, TX in 1991. The objectives of these experiments were 1) to establish the time course of photosynthetic infury in cotton and 2) determine whether changes in carbon exchange rate (CER) can be explained by changes in the chlorophyll fluorescence parameter (Fx/Fp), 1,5-reibulosebisphosphate carboxylase activity (Rubisco), or stomatal conductance (Cond). The first experiment was with cotton cultivar Stoneville 453 on a Weswood silt loam (Fluventic Ustochrept). The second experiment utilized cotton cultivar Deltapine 50 on a Ships clay (Udic Chromustert). Plots were sprayed with 0 or 0.75 lb methomyl/acre on July 8 and 29, 1991. in experiments one and two, respectively. In both experiments, all parameters studied (CER, Fx/Fp, Rubisco, and Cond) decreased with methomyl treatment, but recovered by the fifth day. The changes in CER appeared most closely related to changes in Stomatal conductance. however, CER is likely affected to some degree by the other factors studied.