High-Q transmission high-T(c) superconducting coplanar waveguide resonators have been employed to make accurate measurements of the temperature dependence of the penetration depth via the change in phase velocity. Resonators made from several different film thicknesses are well described using a zero temperature penetration depth and a T-squared temperature dependence using a full-wave spectral-domain model of the coplanar waveguide transmission line. (C) 1993 John Wiley & Sons, Inc.