A theoretical study of the magnetic‐field dependence of the two‐dimensional electron‐gas density, cyclotron mass, and Fermi energy in a modulation‐doped GaAs–Ga1−xAlxAs heterojunction under a magnetic field applied perpendicular to the interface is presented. The study, based on both the envelope‐function approximation and a parametric self‐consistent approach, takes into account band‐nonparabolicity effects. Numerical results show that the above‐mentioned quantities as well as a screening parameter introduced in the calculations are oscillatory functions of the magnetic field. Our results are in good agreement with experimental data previously reported and show that nonparabolicity may significantly influence the behavior of the cyclotron‐resonance parameters. ©1996 American Institute of Physics.