We present a simple yet highly sensitive single-beam experimental technique for the determination of both the sign and magnitude of n(2). The sample is moved along the z direction of a focused Gaussian beam while the repetitively pulsed laser energy is held fixed. The resultant plot of transmittance through an aperture in the far field yields a dispersion-shaped curve from which n(2) is easily calculated. A transmittance change of 1% corresponds to a phase distortion of approximately lambda/250. We demonstrate this method on several materials using both CO(2) and Nd:YAG laser pulses.