Infrared pulsed laser irradiation has been used to generate a fast photosignal from epitaxial YBa2Cu3Ox superconducting films when a small alternating magnetic field is applied in the film plane. No bias current was applied, and no signals were observable in static magnetic fields. The photosignals, 40 ns in duration and tenths of a millivolt in amplitude, are obtained below Tc, with alternating magnetic fields up to 3200 A/m oscillating from 20 to 150 Hz. The laser pulses had a duration of 80 ns, with an absorbed fluence of 1 mJ/cm2 at a wavelength of 10.6 μm. The signal is interpreted in terms of flux motion thermally activated by the laser heating. The magnetic force due to the alternating magnetic field exceeds the pinning force following the absorption of the radiation and generates the fast flux motion observed. The signal dependencies on temperature, magnetic-field amplitude, and absorbed fluence, as well as ac-susceptibility measurements, confirm this explanation. It is shown that this thermally activated motion, generated at j∼jc, is faster than an Anderson-Kim creep. We propose that a thermally activated flux-avalanche model based on the self-organized-criticality theory may describe the photosignal observed.