We present results of electrical and optical measurements in an AlxGa1-xN/GaN heterostructure. The presence of a two-dimensional electron gas at the high-quality AlxGa1-xN/GaN heterointerface is confirmed by Shubnikov-de Haas measurement, which shows well-resolved magnetoresistance oscillations starting in fields below 3 T at 1.3 K. From the temperature dependence of the oscillation amplitude, the obtained effective mass (0.24+/-0.02)m(0) is in excellent agreement with the value of cyclotron resonance measurements in two-dimensional (2D) systems, but larger than the values of theoretical and experimental results in GaN bulk films. We point out that the effective-mass enhancement in 2D systems is due to the effects of band nonparabolicity and wave-function penetration into the barrier material. The results of photoconductivity measurements reveal that persistent photoconductivity (PPC) does exist in the AlxGa1-xN/GaN heterostructure, and that the PPC behavior of AlxGa1-xN/GaN heterojunction is quite different from that of the GaN epitaxial thin films. A possible mechanism is presented to interpret the observed PPC effect. [S0163-1829(98)07843-7].