The electronic nonlinear refractive index in Cs vapor at 1.06 &mgr; is shown to be negative and to consist of two distinct contributions. One of these contributions arises from a two‐photon resonance in the susceptibility and can be made comparable in magnitude to the positiven2in laser glass. The other term arises from population redistributions between the ground and first excited state. We present measurements of self‐defocusing of a single pulse from a Nd: YAG mode‐locked laser that are in agreement with the theoretical predictions.