Growth response of fingerling Heteropneustes fossilis (6.8 ± 0.2 g; 11.2 ± 0.3 cm) to dietary l-leucine levels was assessed by conducting 8-week feeding trial in a flow-through system (1–1.5 L min−1) at 28 °C water temperature. Casein–gelatin-based isonitrogenous (380 g kg−1; crude protein) and isoenergetic [17.9 MJ kg−1; gross energy (GE)] basal diet was supplemented with different levels of l-leucine to achieve desired leucine levels ranging between 10 and 22.5 g kg−1 dry diet. Analysed values were 9.9 (Lc9.9), 12.4 (Lc12.4), 15.1 (Lc15.1), 17.4 (Lc17.4), 20.1 (Lc20.1) and 22.4 (Lc22.4) g leucine kg−1 diet. Fishes were stocked randomly in quadruplicates and fed to satiation at 07:00 and 17:30 h. Maximum absolute weight gain (AWG g fish−1), feed conversion ratio (FCR), protein utilization efficiency (PUE%), leucine retention efficiency (LRE%) and haematological parameters were found in fish fed diet Lc17.4. For precise determination of dietary leucine requirement of Singhi, AWG g fish−1, FCR, PUE% and LRE% were subjected to broken-line and second-degree polynomial regression analysis. Second-degree polynomial regression analysis fitted the data more accurately (P > 0.05) exhibiting high R2 values. Hence, based on this analysis, dietary leucine requirement of fingerling H. fossilis is recommended to be 16.5 g kg−1 of the diet, corresponding to 43.4 g kg−1 protein for developing leucine-balanced commercial feeds.