Cotton (Gossypium hirsutum L.) varieties containing a gene that encodes an insecticidal protein were released commercially in 1996. The insecticidal protein encoded by the gene proved especially effective in reducing damage to cotton plants from lepidopteran pests by using limited additional pesticide applications. The gene was transferred from the bacterium Bacillus thuringiensis var. kurstaki to the cotton variety Coker 312. Coker 312 is considered to be an extremely vigorous variety that exhibits rank vegetative growth. Therefore, Bt cotton varieties exhibit rank vegetative growth similar to their recurrent parent Coker 312. Traditionally, mepiquat chloride (1,1-dimethylpiperidinium chloride) has been applied to suppress rank vegetative growth. Mepiquat chloride (MC), a plant growth regulator, reduces main stem elongation of cotton plants. Current crop simulation models estimate that the optimum concentration of MC needed to reduce main stem elongation lies between 10 and 12 ppm. However, transgenic varieties, such as the Bt varieties, may behave differently toward MC than their conventional counterparts. To test this hypothesis, a field experiment was conducted at the Texas Agricultural Experiment Station in Corpus Christi, Texas during the 1998 growing season. The factorial field experiment was arranged as a randomized complete block design with four replications. The experiment consisted of 2 varieties and 5 variable rates of MC. The varieties planted included a Bt variety (DPL 33^B) and the same variety without the Bt gene (DPL 5415). Pix^® (4.2% MC) was applied in a single application near the 12-node stage at the following rates: 5, 10, 15, and 20 oz/A. Weekly height, node number, and dry weight determinations were collected. In addition, plots were harvested by hand twice to determine yield. Data was analyzed using regression, ANOVA, LSD, and analysis of covariance. Significant differences in plant height were observed between the different MC rates. The control plants were the tallest, followed by plants treated with the 5, 10, 15, and 20 oz MC/A rates, respectively. Analysis of covariance results detected no significant differences in the growth rate between the Bt and the conventional varieties. However, the Bt variety was numerically taller than the conventional variety in the control, 5, and 10 oz MC/A rates. Furthermore, no significant differences in final plant height were observed between the two varieties. The MC-height reduction model developed in this study was similar to an existing MC concentration-height reduction model. In addition, no significant differences in yield were observed between the Bt and the conventional varieties. However, the Bt variety was significantly earlier than its conventional counterpart. A significant reduction in total yield was observed for the 20 oz MC/A application rate. Thus, we concluded that Bt varieties behave similarly with respect to vegetative growth to applications of MC as their conventional counterparts. Therefore, current MC application rate strategies should suffice for suppressing rank vegetative growth of Bt cotton.