Magnetophotoluminescence has been studied from a single undoped GaN/AlxGa1-xN heterojunction with a linewidth of 2.5 meV. The peak originates from the recombination of a photoexcited hole with an electron in the two-dimensional electron gas (2DEG) formed as a result of spontaneous and piezoelectric polarizations at the interface. The photoluminescence intensity is strongly enhanced at filling factors corresponding to filled Landau levels as a result of the reduced screening of the Coulomb interaction by the 2DEG. This prevents the rapid diffusion of photoexcited holes away from the heterojunction. The energy of the magnetoexcitonic recombination indicates a very low value for the hole mass of 0.3m0 close to the band edge in agreement with theory.