Application of the insecticide methomyl frequently results in reddened foliage and early season defoliation of Gossypium hirsutum L. Experiments utilizing field-grown cotton were conducted to investigate methomyl's influence on cotton leaf senescence. Gas chromatographic measurements of ethylene evolution from sampled leaf discs after application of 0.28 kg a.i./ha methomyl were significantly elevated for 48 h over the untreated control. Leaf disc conductances indicating membrane permeability were 51.7 and 33.7 µ amps current for 0.28 kg/ha methomyl and the control, respectively. Potassium, calcium, and nitrogen levels in the leaf disc leachates from the 0.28 kg/ha methomyl treat ment and the untreated control were 45.1, 19.1, 11.8, and 24.6, 10.6, 5.2 ppm, respectively. Two applications of 0.56 kg a.i./ha methomyl resulted in reddened tissue that contained significantly higher anthocyanin and lower chlorophyll levels compared to green tissue of the control plants. Oxygen uptake of leaf discs, a measure of respiration, was 3.43 and 2.92 ppm, respectively. Significant increases in ethylene evolution, membrane permeability and respiration, and decreased chlorophyll levels resulting from methomyl application suggest that treatment with the insecticide stimulates premature cotton leaf senescence.