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Plant growth regulators are routinely used in cotton (Gossypium hirsutum L.) production to reduce plant height and hasten maturity. The objective of this research was to determine the response of several cotton cultivars to mepiquat pentaborate (MPB) application in environments accumulating different levels of heat units. Four MPB application regimes were imposed on three cultivars in Virginia and South Carolina in 2005 and 2006. Total MPB season rates of 0.0, 54.9, 85.3, or 121.9 g ai ha -1 applied at the five-leaf stage, pin-head square, match-head square, and early bloom were used. The cultivars were: Deltapine (DP) 444 BG/RR, an "early-maturing" cultivar; Fibermax (FM) 960 BR, a "medium-maturing" cultivar; and DP 555 BG/RR, a "late-maturing" cultivar. In South Carolina in 2006, FM 960 BR July plant height was reduced by 25% with MPB application compared to only 12 and 13% for DP 444 BG/RR and DP 555 BG/RR, respectively, although actual plant height reductions were not different among cultivars. Mepiquat pentaborate applications decreased plant height at harvest by 8 to 34%, height-to-node ratio by 10 to 32%, enhanced maturity as measured by nodes above white flower for all cultivars, and decreased lint yield by 3.7 to 8.5% compared to untreated cotton. Higher seasonal totals and earlier initiation of MPB application resulted in the greatest decrease in lint yield.