Crystals of &agr;‐[(4’‐methoxyphenyl)methylene]‐4‐nitro‐benzeneacetonitrile exhibit powder second‐harmonic generation (SHG) efficiencies under 1.06 &mgr;m irradiation several times that of 2‐methyl‐4‐nitroaniline. We have identified three distinct, solution‐grown polymorphs which show marked variation in SHG efficiency. Controlled transformation between these polymorphs can be made either solvent assisted or thermally. One polymorph shows a 25‐fold increase in SHG efficiency when the irradiation wavelength is changed to 1.32 &mgr;m while the others show a small or no increase.