The penetration depth and surface resistance of a superconductor depend upon the superfluid current density. This dependence gives rise to nonlinear mixing in a superconducting microstrip resonator. Here we discuss the problem of intermodulation in which two signals at&ohgr;1and&ohgr;2, laying within the pass band of a microstrip cavity resonance, mix and generate a signal at2&ohgr;1−&ohgr;2. An expression relating the power generated at2&ohgr;1−&ohgr;2to the power transmitted at&ohgr;1and&ohgr;2is given. We focus on the high-Tcsuperconductors where it is believed that the order parameter hasdx2−y2symmetry. We find for a resonator with a large unloadedQthat intermodulation arises dominantly from the reactive nonlinear inductance of the superconducting film. ©1997 American Institute of Physics.