Landau-level oscillations are observed in the photoluminescence from an AlxGa1-xAs-GaAs single heterojunction in an applied magnetic field. Extrapolating the oscillations back to zero field gives the energy of the transition from the two-dimensional (2D) electrons to the free heavy holes (hh1 and hh2) and free light holes (lh) weakly confined in the GaAs active layer. The measured energy separation between hh1 and lh is 1.8 meV, which agrees very well with the calculated value of 2.2 meV. The measured energy separation between hh1 and hh2 is 3.8 meV, in good agreement with the calculated value of 4.4 meV. The 2D electron effective mass was determined to be me*=0.086m. The light-hole effective mass was determined to be mlh*=0.078m, using the GaAs heavy-hole effective mass mhh1*=0.46m as input. The effective mass mhh2* is approximately the same as mhh1*.