We report the direct autocorrelation of ultrashort mid-infrared laser pulses, using an unbiased GaAsy/GaAlAs quantum-well device as a nonlinear photovoltaic medium. Using this method, we autocorrelate ultrashort (3-ps) low-energy (~500-nJ) pulses of 10.6-microm light. This approach should permit the autocorrelation of subpicosecond infrared pulses.