Cotton yield components and fiber quality were examined in a 2-yr field study with four rates of fertilizer-K (mainplots) and two cultivars as subplots. Lint yields of both cultivars increased linearly with the level of K addition. Yield increases of 35% and 54% were obtained at the highest K-rate in 1986 and 1987, respectively. Higher lint yield with added K resulted from a higher lint percentage and increased boll retention, especially at later fruiting positions. Increased soil K availability and plant K content also improved fiber quality including fiber length, micronaire, strength, elongation percent, and uniformity. Regression of 2 each quality parameter on (i) lint K concentration (r² values from 0.21 to 0.60), (ii) leaf K concentration at early bloom (r² values from 0.29 to 0.49), or (iii) K concentration in soil solution (r² values from 0.21 to 0.54) indicate that K supply to developing fruit is an important determinant of fiber growth. For each quality parameter, regression lines for 'Acala GC510' had higher y-intercepts and smaller slopes than for "Acala SJ-2'. This suggests that cultivars differ in their ability to produce acceptable lint quality under K limiting conditions.