Two field experiments were conducted at Sakha Agricultural Research Station, ARC, Kafr El-Sheikh, Egypt, in the 2017 and 2018 seasons to study the effect of growth regulators (PGRs) foliar application on cotton fiber growth and development (elongation and maturation), using Giza 93 (ELS) and Giza 94 (LS) Egyptian cotton varieties. Treatments of PGRs were made using tap water to final concentrations of 50, 100, and 150 ppm of Indol Butyric Acid (IBA) and Mepiquat Chloride (MC), while Gibberellic Acid (GA3) concentrations were 100, 200, and 300 ppm. These treatments were performed at the beginning of flowering (60 days after planting). The experimental design was a split plot design in three replicates. The obtained results indicated that PGRs treatments showed significant effects on fiber growth in length, elongation rate, and secondary wall thickening along boll ages. In both seasons, 100 ppm GA3 treatment followed by 100 ppm IBA showed the highest increase in fiber elongation, elongation rate, and increased halo length in the two varieties. While MC (pix) treatment showed insignificant increases and decreases in fiber length and halo length of the two varieties. MC and IBA treatments surpassed GA3 treatments in increasing the secondary wall thickening, especially at 50 ppm of MC. GA3 and IBA treatments exerted an overlap between elongation and cellulose deposition phases. The growing season significantly affected the degree of thickening significantly, whereas its effect on fiber elongation and its rate was of low magnitude. Season and PGR treatments had no effect on fibre perimeter.