Measurements, using a bimodal cavity, of the temperature and magnetic field dependence of the surface impedance of a single crystal of superconducting tin at a frequency of 23.5 Gc/sec are reported. In the geometry used, the static and microwave magnetic fields were parallel over the surface of the sample. The temperature dependence of the reactance in zero field agrees well with that calculated by Miller from the BCS theory. A small unexpected knee in the temperature dependence of the resistance is observed, centered around a reduced temperature of $t=0.88$. This knee may arise from a band of $p$ electrons with a smaller energy gap whose temperature dependence differs markedly from the BCS form, as proposed by Suhl, Matthias, and Walker. The change of surface resistance with applied field is found to be negative in the temperature range of the knee with the greatest negative dependence occuring at the center of the knee. The change of resistance is found to be positive outside this temperature range. The change of reactance with field is found to be positive. The association of the negative field dependence of the surface impedance with a band of $p$ electrons is shown to give a qualitative explanation of the field dependence as observed in this and other investigations.