Ethephon (2-chloroethyl phosphonic acid) may interact with various defoliants in cotton (Gossypium hirsutum L.), because of differences in mode of action or optimal temperatures for activity. Field temperatures for harvest aids are unpredictable, but later plantings are more likely to encounter cooler ambient temperatures than earlier plantings. We planted 'Deltapine 50' cotton in late April and mid-May for 3 years at the West Tennessee Experiment Station to study harvest-aid responses under different temperature regimes. Defoliants were tribufos (S,S,S,-tributyl phosphorotrithioate); dimethipin (2,3-dihydro-5,6-dimethyl-1,4-dithiin 1,1,4,4-tetraoxide); thidiazuron (N-phenyl-N'-1,2,3-thiadiazol-5-ylurea), and an untreated check, applied at 53 ± 5% open bolls with and without ethephon. Cumulative degree-days after treatment (base 15.6°C) to final harvest ranged from 86 to 147 in the early plantings, and from 37 to 61 in later plantings. Interactions affecting defoliation, boll opening, and lint yields were more common under cooler conditions of the later plantings. Ethephon increased defoliation with thidiazuron more than with tribufos or dimethipin under cooler conditions. Tribufos or dimethipin defoliated faster than thidiazuron under cooler temperatures. Boll opening response to ethephon was reduced by tribufos in two warmer tests, and it was reduced by dimethipin in cooler conditions, relative to thidiazuron. Across defoliants, ethephon increased total lint yields in two tests under cooler conditions, but it decreased yields slightly in the warmest test. Optimum defoliation and boll opening responses require the use of different defoliants with ethephon as field temperatures change.