[en] The surface resistance R of two UPt₃ single crystals has been measured using an apparatus in which the direction of the rf or microwave current can be varied continuously relative to the crystal axes. In an a-axis single crystal at 0.38 K, R is found to be lower for current in the basal plane than for current along the c axis, while R is independent of current direction in a c-axis crystal. This suggests that the superconducting-energy-gap function is anisotropic with, on average, a larger gap in basal-plane directions than in the c-axis direction. R at 0.38 K is found to increase anomalously rapidly with increasing frequency to nearly the normal-state value at frequencies that are only a few percent of kT_c