Foliar application of Promote 125, a carboxylic-acid-based plant growth regulator, at the time when cotton plants are heading towards cut-out, may delay the onset of internal energy/substrate deficits long enough to produce one or two more bolls and/or bigger bolls. The objective of this study was to evaluate the effects of application rates of the plant growth regulator Promote 125 on growth, yield, and fiber quality of cotton grown under deficit irrigation and dryland conditions. The study was conducted at the Texas A&M University Agricultural Research and Extension Center in Corpus Christi, TX, during the 2000 and 2001 seasons. Treatments included two rates of Promote 125: 2.0 oz./c and 4.0 oz./ac applied at 7 nodes above white flower. Promote 125 applied at 2.0 oz./acre appears to be a promising plant growth regulator to increase yields in cotton. The magnitude of yield increases over the untreated controls measured in 2000 under deficit irrigation and in 2001 under dryland conditions were important; 173 lbs./acre (13% increase) and 165 lbs./acre (17% increase), respectively. The yield component associated with yield increase could not be clearly identified from these experiments. While the yield increase observed in 2001 resulted from a significant increase in the number of bolls per plant, this effect was not observed in 2000. Boll size (lint per boll) was not a factor in the yield increase resulting from the applying Promote 125 at 2 oz./ac.