We report our femtosecond time-resolved measurements on the photoresponse of an epitaxial YBa2Cu3O7−x (YBCO) thin-film photodetector, patterned into a microbridge geometry. By varying the current–voltage biasing conditions between the superconducting and resistive (hot spot) states, we observed transients that correspond to the nonequilibrium kinetic-inductance and the nonequilibrium electron-heating response mechanisms, respectively. The two-temperature model and the Rothwarf–Taylor theory have been used to simulate the measured wave forms and to extract the temporal parameters. The electron thermalization time and the electron–phonon energy relaxation time were determined by the electron temperature rise and decay times, which were found to be 0.56 and 1.1 ps, respectively, in the resistive state. We have also measured the ratio between the phonon and electron specific heats to be 38, which corresponds to a phonon–electron scattering time of 42 ps. No phonon-trapping effect (typical for low-temperature superconductors) was observed in YBCO, in the superconducting state, so the quasiparticle lifetime was given by the quasiparticle recombination time, estimated from the Rothwarf–Taylor equations to be below 1 ps. © 1999 American Institute of Physics.