Optimum level of dietary lysine for fingerling rohu, Labeo rohita (4.42 ± 0.1 cm; 0.66 ± 0.08 g), was determined by feeding isonitrogenous (40% crude protein) and isocaloric (4.28 kcal/g) diets with graded concentrations of lysine (1.20, 1.40, 1.60, 1.80, 2.0, and 2.20% dry diet) to triplicate groups of fish in 80-L circular polyvinyl troughs (water volume 55 L) fitted with a continuous flow-through system (1–1.5 L/min) for 12 wk. Growth performance was evaluated on the basis of absolute weight gain (AWG g/fish), feed conversion ratio (FCR), protein efficiency ratio (PER), and protein retention efficiency (PRE%). Adequacy of dietary lysine was estimated using broken-line and nonlinear regression models. When assessed using broken-line model, these parameters were best at 1.82, 1.81, 1.80, and 1.81%, respectively, dietary lysine, corresponding to 4.55, 4.52, 4.50, and 4.52%, respectively, of the dietary protein, whereas when subjected to second-degree polynomial regression analysis optimum dietary lysine was found at 1.91, 1.90, 1.91, and 1.91% of dry diet, corresponding to 4.80, 4.75, 4.80, and 4.75%, respectively, of the dietary protein. Based on the above regression analyses, optimum dietary lysine requirement for fingerling L. rohita was found to range between 1.80 and 1.91% of the diet, corresponding to 4.50–4.80% of dietary protein.