Abstract Crop growth and development are directly associated with the thermal regime (heat-unit concept), when other environmental factors are not limiting. The growing-season length on the Texas High Plains is less than optimal for cotton production, based upon total available heat units. In addition, water stress, due to inadequate rainfall, coupled with the high evaporative demand, represents the major environmental contraint to cotton yield. A field experiment was conducted to evaluate the relationship between cotton lint yield and available heat units in this climate. When water supply was not the major limitation to productivity, lint yield was highly correlated with the available heat units during the growing season ( r 2 =0.90). When water stress represented the major growth constraint (seasonal water supply less than 400 mm), no significant relationship existed between lint yield and seasonal heat-unit accumulation. Evaluation of the response of specific growth stages revealed that the accumulation of heat units during the early fruiting period explained as much of the yield variability as did the seasonal heat-unit accumulation.