In 1982 and 1983 a field experiment was conducted in a Bayamón clay soil on a farm near Arecibo, Puerto Rico, to evaluate the effect of rates of K fertilization (0, 45, 136 and 272 kg/ha/year) on dry forage (DF), crude protein (CP) content and CP yields, K content and K uptake, and P, Ca, and Mg content. Six grasses (Cynodon dactylon cv. Coastcross-1, Digitaria penizii cv, Slenderstem, C. datylon cv. toño, D. decumbens cv. Transvala, D. nlemfuensis cv. Star and D. decumbens cv. Pangola) were cut at 45-day intervals. The mean of yields of the first four grasses, 37,200 kg/ha, were significantly higher (P = 0.05) than those of the other two in 1982. In 1983 the first grass was susceptible to weed infestation. The mean DF yields of the other grasses, 19,499 to 21,198 kg/ha, were significantly higher (P = 0.05) than those of C. nlemfuensis cv. Star. The 136 and 272 kg/ha/ year K rates produced significantly higher DF yields in 1982, but in 1983 only the highest rate had any signficant effect (P = 0.05). The CP content of D. decumbens cv. Pangola, 12.29%, was significantly higher (P = 0.05) than that of the others in 1982, but in 1983 C. nlemfuensis cv. Star showed a signficantly higher (P = 0.05) CP content, 9.85%. Potassium rates did not have a significant effect (P=0.05) on CP content. CP yields were related to DF yields and followed similar patterns both years. The mean K content of all grasses was 1.75 and 1.53%; and K uptake was 603 and 299 kg/ha/ year, in 1982 and 1983, respectively. These values were not significantly different (P = 0.05) among species but they were significantly different (P= 0.05) for the two highest K rates in 1982, and for the highest in 1983. The mean P content, 0.29%; Ca content, 0.46% and Mg content, 0.37%, were similar for all grasses and were not affected by K treatments.