In 1996, cotton (Gossypium hirsutum L.) cultivars expressing the Bacillus thuringiensis (Bt) toxin gene were introduced commercially. These transgenic varieties revolutionized crop production by controlling some lepidopteran pests with limited additional pesticide applications. However, these transgenic plants also experience rank vegetative growth. Crop simulation models currently estimate the effects of mepiquat chloride (MC) [1,1-dimethylpiperidinium chloride], tradename Pix, on main stem elongation and leaf area expansion rates for non-transgenic varieties. Transgenic plants, such as Bt cultivars, may behave differently to applications of mepiquat chloride than their non-transgenic counterparts. In addition, more MC may be required to control the excess vegetative growth of these varieties. To test these hypotheses, a field study was performed involving variable MC application rates. Pix (4.2 % mepiquat chloride) was applied near match-head square in a single application at the following rates: 5, 10, 15, and 20 oz/A. Data collected included the following weekly measurements: plant height, number of main stem nodes, average top 5 internode length, and total plant dry weight. Significant differences in plant height were observed between MC application rates. In addition, a regression model (R²=0.80) was developed relating plant height to the calculated MC concentration of the plant tissue. The model developed for the Bt cultivar NuCOTN 33^B is similar to the MC height model devised for a non-transgenic variety (DPL 50). Therefore, we concluded that transgenic and non-transgenic cotton plants respond similarly to mepiquat chloride.