We show that the nonlinear index perturbation due to light‐induced photovoltaic space‐charge field in LiNbO3can give rise to self‐phase matching of second harmonic generation. Increase of the conversion efficiency is accompanied by formation of stationary and nonstationary patterns in the spatial structure of the generated second harmonic. The space‐charge field can be induced either by the initially non‐phase‐matched second harmonic or by an external seed. ©1996 American Institute of Physics.