Cyclotron resonance studies of two-dimensional electrons confined at the GaN/AlGaN interface are presented. The value of the 2D-electron cyclotron mass is determined and discussed in view of nonparabolicity and polaron effects. The influence of nonparabolicity is enhanced by the spatial confinement of electrons and is calculated in the triangular well approximation. After subtraction of nonparabolicity corrections, the same polaron mass (0.223±0.011)m 0 is obtained for 2D and bulk electrons in GaN.