Two field experiments were carried out at Giza Agricultural Re­search Station during 1997 and 1998 seasons on Giza•85 cv. to study the response of cotton plants to the natural auxin indole acetic acid (IAA) during the flowering period. IAA was applied at start of flowering at rates of 0, 25, 50, 75 and 100 ppm as foliar spray. The obtained results revealed that IAA application affected signif­icantly most of the chemical constituents of cotton leaves i.e., chloro­phyll a, total soluble sugars and phenolic compounds especially poly-phenols. Changes in levels of promoters and inhibitors in cotton leaves were studied. Auxin-treated plants contained less levels of inhibitors and application of 75 and 100 ppm IAA exerted higher concentration of pro­moters which enhanced the vegetative growth and resulted in significant increases in plant height and dry matter production. IAA application of 50 ppm was resulted in plants producing higher numbers of flowers and total bolls per plant as well as higher boll setting percentage. Applying 100 ppm IAA produced less number of open bolls/plant. Boll weight, seed index, earliness percentage were increased by IAA application. Ap­plication of 50 ppm of IAA increased significantly seed cotton yield (ken­tar/feddan), however application of 100 ppm produced less yield as high concentration induced the vegetative growth on the expense of fruiting growth. However, lint percentage and fiber properties showed no signifi­cant response due to IAA application.