The effects of dietary Na/K ratios on feed intake, growth, nutrient utilisation, plasma and body mineral concentration in juvenile African catfish were investigated. Four Na/K ratios, 0.2, 0.7, 1.5, 2.5 (mmol/mmol), were tested in a randomised experimental design with three replications in each treatment. Juvenile African catfish (n=360) with a mean initial body weight of 42.9 g were used. The experiment was conducted in 12 (70 l) aquaria with a biofilter and recirculating water supply system. Dietary Na/K ratios were obtained by altering levels of Na2CO3 and K2CO3. Feed was provided manually twice a day to satiation. Feed consumption was not influenced by the mineral composition of the diets. Growth, protein retention, nitrogen and energy utilisation efficiency increased quadratically, body dry matter, fat, nitrogen and energy content increased in a linear or quadratic manner with increasing dietary Na/K ratio. Feed conversion and percentage of energy retention as protein decreased in a quadratic manner. Potassium and chloride concentrations and the ratios between Na/K, Na/Cl and K/Cl in the body remained constant among treatment groups at the end of a 4-week feeding period. In conclusion, excess K in the diet depresses growth and nutrient utilisation efficiency, reduces body fat and protein deposition. The dietary Na/K ratio of 1.5 to 2.5 produced the best growth of African catfish.