To quantify dietary L-tryptophan requirement of fingerling Heteropneustes fossilis (6.66 ± 0.08 g), casein–gelatin-based isonitrogenous (38% CP) and isoenergetic (14.72 kJ g−1 DE) purified diets with eight levels of L-tryptophan (0.12%, 0.16%, 0.20%, 0.24%, 0.28%, 0.32%, 0.36%, 0.40% dry diet) were fed to triplicate groups of fish twice daily to apparent satiation for 12 weeks. Incremental levels of dietary tryptophan from 0.12 to 0.28% significantly (P < 0.05) improved absolute weight gain (AWG; 14.3–65.9 g fish−1), feed conversion ratio (FCR; 5.9–1.5), protein retention efficiency (PRE; 6.2–32.2%), haemoglobin (Hb; 6.5 to 11.9 g dL−1) and haematocrit (Hct; 23.5–33.8%). To determine the precise information on tryptophan requirement, data were subjected to broken-line and second-degree polynomial regression analysis. Broken-line regression analysis reflected highest R2 values for AWG g fish−1 (0.999), PRE% (0.993), Hb g dL−1 (0.995) and Hct% (0.993) compared with R2 values obtained using second-degree polynomial regression analysis of AWG g fish−1(0.949), PRE% (0.890), Hb g dL−1(0.969) and Hct% (0.943), indicating that data were better fit to broken-line regression analysis. Hence, based on broken-line regression analysis at 95% maximum response, tryptophan requirement of fingerling H. fossilis is recommended between 0.24% and 0.27% dry diet (0.63–0.71% protein).