Six treatments of Ginstar (Thidiazuron + Diuron) and one of Dropp (Thidiazuron) were evaluated at two maturity stages (15 y 70% boll opening) in 1995, on cotton Cv. DELTAPINE 5415, to evaluate its effect as an aid to reduce feeding sites for whitefly, especially when boll opening coincides with the peak of whitefly (Bemisia argentifolii). The experiment was conducted in the Yaqui Valley Experiment Station in the state of Sonora, Mexico. Early defoliation (when only 15% of bolls are open) reduced yield, and most yield and fiber quality components, but the difference was not significant. Defoliation with Ginstar in doses of 150 cc/ha or Dropp at 15 gr/ha, did not affect lint and seedcotton yield in DELTAPINE 5415. Doses of Ginstar of 250 cc/ha or larger, reduced lint yield and seedcotton at first pick, but the difference was not significant. Defoliation with 150 cc of Ginstar at 70% of boll opening, induced a higher seedcotton yield at first pick, and slight increase in fiber strength, in comparison with the check, but again the differences were not significant. No clear trend was observed in regard to boll size, seed index, and micronaire index as a result of early defoliation and higher doses in either defoliant product.