Variation in the growth habit and maturity of cotton (Gossypium hirsutum L.) cultivars is a complicating factor in cultivar testing, which necessitates separate tests for early and full season cultivars. In conventional production systems, commercial cultivars often have a chemical growth regulator applied, but the interaction of these regulators with cotton cultivars of different maturities is not fully understood. This study evaluated the use of mepiquat pentaborate on a group of commercially available cultivars that differ in growth and maturity. Treatments were mepiquat pentaborate at 115 g a.i. ha-1 and 230 g a.i. ha-1 each applied in two sprayings during the season, and an untreated check. Treatments were applied to all cultivars on the same calendar dates, regardless of developmental stage. Four early and four late maturing cultivars were evaluated in 2003 and 2004 for the effects of mepiquat pentaborate on yield, yield components, and fiber properties. Analyzed across years, data showed significant interactions between mepiquat pentaborate treatments and cultivars for lint yield and lint percentage that resulted in a positive lint yield response in one cultivar, accompanied by varying reductions in lint percentage for all cultivars. Application of mepiquat pentaborate caused reductions in plant height, height to node ratio, and nodes above white flower. Mepiquat pentaborate treatments made small, but statistically significant improvements on fiber length, strength, micronaire, uniformity, and short fiber content. Significant variation in the short fiber content among cultivars was also detected. The results of this study support the perception that yield interactions may occur when plant growth regulators are used in cultivar testing.