A growth trial was conducted to estimate the optimum concentration of dietary potassium (K) for grass carp (Ctenopharyngodon idella). Triplicate groups of grass carp (3.96 ± 0.06 g) were fed diets containing graded levels (0.87, 2.90, 5.37, 7.54, 9.87 and 12.4 g kg−1) of K for 8 weeks. Final body weight, weight gain and feed efficiency and gill Na+-K+ ATPase activity were highest in fish fed with 9.87 g kg−1 dietary K and lowest in fish fed the basal diet (P < 0.05). The K contents in whole body and muscle were linearly increased up to the 9.87 g kg−1 dietary K and then levelled off beyond this level, whereas in scales and vertebrae up to the 7.54 g kg−1 dietary K (P < 0.05). However, dietary K levels had no significant effect on ash, Ca, P and Mg contents in whole body, scales, vertebrae or muscle. Analysis using polynomial regression of weight gain and gill Na+-K+ ATPase activity and using the broken-line regression of whole body K concentrations indicated that the adequate dietary K concentration for grass carp is about 9.45–9.99 g kg−1 diet.