The effect of two digestible protein levels (310 and 469 g/kg DM) on the relative lysine (Lys; g Lys/kg DM or g Lys/100 g protein) and the absolute Lys (g Lys intake/kg0·75 per d) requirements was studied in rainbow trout fry using a dose–response trial. At each protein level, sixteen isoenergetic (22–23 MJ digestible energy/kg DM) diets were tested, involving a full range (2–70 g/kg DM) of sixteen Lys levels. Each diet was given to one group of sixty rainbow trout fry (mean initial body weight 0·78 g) reared at 15°C for 31 feeding d. The Lys requirements were estimated based on the relationships between weight, protein, and Lys gains (g/kg0·75 per d) and Lys concentration (g/kg DM or g/100 g protein) or Lys intake (g/kg0·75 per d), using the broken-line model (BLM) and the non-linear four-parameter saturation kinetics model (SKM-4). Both the model and the response criterion chosen markedly impacted the relative Lys requirement. The relative Lys requirement for Lys gain of rainbow trout estimated with the BLM (and SKM-4 at 90 % of the maximum response) increased from 16·8 (19·6) g/kg DM at a low protein level to 23·4 (24·5) g/kg DM at a high protein level. However, the dietary protein content affected neither the absolute Lys requirement nor the relative Lys requirement expressed as g Lys/100 g protein nor the Lys requirement for maintenance (21 mg Lys/kg0·75 per d).