The effects of conduction‐band nonparabolicity on quantized energy levels of an In0.5Ga0.5As /In0.5Al0.5As quantum well have been analyzed using the original Luttinger–Kohn ‘‘effective mass’’ equation, which is, in principle, valid as long as the perturbation to the periodic lattice potential can be regarded as slowly varying. The results differ substantially from those previously reported which employed the energy‐dependent effective mass approach.