We measure the spectral phase of femtosecond optical pulses using a time-frequency analog of Young's doubleslit interference. A pair of narrow slits in an opaque sheet selects two spectral frequencies from the femtosecond pulse spectrum in a zero-dispersion pulse stretcher. Measurement of the temporal phase of a family of beat frequencies obtained over a range of slit spacings yields the desired spectral phase directly. We demonstrate this technique by accurately measuring the quadratic phase added to 80-fs optical pulses by a 6.5-cm block of BK-7 glass.