Nonlinear refraction in a number of crystals (KGW, KGW:Eu^3+, Ba(NO_3)_2, BaWO_4, PbMoO_4, and CdWO_4) that are promising for excitation of multifrequency stimulated Raman scattering is investigated by a single-beam Z-scan method. Picosecond laser pulses with a wavelength of 1064 nm generated by a YAG:Nd^3+ laser were used for excitation of nonlinear optical processes. The values of n _2 are compared depending on the type of the crystal, optical-axis orientation, and impurity concentration.