Abstract: In order to investigate the interaction of NH4+ with K+ in cotton seedlings, an hydroponics experiment containing three K+ concentrations (0.03, 0.5, 2.5 mmol/L) combined with four NH4+ concentrations (0, 0.5, 2.5, 5.0 mmol/L) was conducted to examine the responses of seedling growth, K+ uptake and utilization to NH4+ with two cotton genotypes, Lumianyan 22 and 153018, differed in potassium efficiency in greenhouse. The results indicated that NH4+ significantly inhibited seedling growth, K+ uptake and utilization irrespective of K+ level in solution. However, K+ at 0.5 and 2.5 mmol/L concentration could alleviate the impact of NH4+ similarly, compared to 0.03 mmol/L K+. For example, dry matter of Lumianyan 22 was reduced to only 76.5% of control by 0.5 mmol/L NH4+ under 0.03 mmol/L K+, and to 88.4% and 89.2% of control under 0.5 and 2.5 mmol/L K+ respectively. In addition, the magnitude of the NH4+ effect differed among genotypes. Lumianyan 22, more sensitive to K+ deficiency than 153018, was affected by NH4+ to a larger extent than the latter. Further analysis showed that NH4+ had the same effect on K+ accumulation of Lumianyan 22 and 153018, but reduced Lumianyan 22 K utilization index more than the latter, which may have contributed to higher sensitivity of Lumianyan 22 to NH4+, compared to 153018. Nevertheless, we can not conclude that there is inevitable correlation between K+-sensitivity and NH4+-sensitivity in cotton seedlings based on the results of this study.