Twenty-one semipurified diets were formulated to determine the optimum protein:energy ratio (P/E) for channel catfish fingerlings. Seven crude protein levels and three energy levels at each protein level were utilized. The protein, lipid, and digestible carbohydrate sources were hexane-extracted whole egg powder, salmon-corn oil mixture, and white dextrin, respectively. After an initial 2 week conditioning period, the experimental diets were fed in triplicate to groups of 30 +/- 3 channel catfish fingerlings weighing 205 +/- 10 g and kept in 110 liter flow-through aquaria at 26.7 +/- 1.1 degrees. The diets were fed at a rate equalling 3% of the total wet body weight/day divided into three equal feedings and adjusted weekly for 9 weeks. An approximate fivefold increase in body weight was observed. These data indicate that the concept of P/E ratios must be restricted to diets containing adequate levels of protein and total energy. Based on average daily gain, diets containing energy levels between 275 and 341 kcal/100 g had an optimum P/E ratio of approximately 88 (24-36% crude protein). Diets containing 24% crude protein and 275 kcal/100 g diet appeared to be utilized more efficiently in terms of percent protein deposited than diets containing higher crude protein levels.