Late planting causes the crop to flower later and pushes boll development into the cooler weather, resulting in reduced yields. Potassium deficiency associated with the 0 kg K treatment elicits some of the same responses in cotton as delayed planting. Together, these stresses may affect the yield and quality beyond the individual effects of late planting and K deficiency. The objective of this research was to determine the effects of planting date and K fertilization on yield and fiber properties of cotton. The experiment was designed as a split-plot with four replications with planting dates as the main plots. Subplots were levels of potassium. Data collected in 2000 and 2001 indicate that planting date and K fertilization produced significant differences in seed cotton yield, lint yield and earliness. Potassium application in early planted cotton gave the highest boll weight while potassium deficiency related with the 0 kg K treatment produced the lowest boll weight. On average, the best yield response of cotton to planting date treatments was when the crop was sown early. Lint yield was 11.2% higher for the early planting date than for the late planting date. The K fertilization rate had an impact on earliness only in 2001. Averaged across years, soil applications of 150 kg K2O/ha decreased the number of days required from planting to open first boll 1.8% compared with the no potassium control treatment. Late planted cotton without the K treatment gave the greatest number of days to boll opening. It is concluded that cotton grown in this region can benefit from K treatments. While current soil K rates are sufficient, reductions in lint yield and some fiber quality characteristics may result if sowing occurs after mid-May. (C) 2002 Elsevier Science B.V. All rights reserved.