Two-photon absorption coefficients /32 of ten direct gap semiconductors with band-gap energy Eg varying between 1 .4 and 3.7 eV were measured using 1.06 µm and 0.53 um picosecond pulses. 2 was found to scale as E43, as predicted by theory for the samples measured. Extension of the empirical relationship between 2 and Eg to InSb with Eg = 0.2 eV also provides agree-ment between previously measured values and the predicted 02. In addition, the absolute values of $2 are in excellent agreement (the average difference being <26%) with recent theory, which includes the effects of nonparabolic bands. The nonlinear refraction induced in these materials was monitored and found to agree well with the assumption that the self-refraction originates from the two-photon-generated free carriers. The observed self-defocusing yields an effective nonlinear index as much as two orders of magnitude larger than CS2 for comparable irradiances. This self-defocusing, in conjunction with two-photon absorption, was used to construct a simple, effective optical limiter that has high transmission at low input irradiance and low transmission at high input irradiance. The device is the optical analog of a Zener diode.