The organic crystal 4‐nitro‐4’‐methylbenzylidene aniline (NMBA) was identified as a promising nonlinear material by the powder technique. The material gave a second harmonic intensity 16 times that of urea. Large single crystals of dimensions 5×3×1 cm3were grown by the temperature lowering of a seeded supersaturated ethyl acetate solution. The principal dielectric axes were defined by orthoscopic examination. The dispersions of the refractive indices were determined to an accuracy of ±0.0015 using the minimum deviation technique and Maker fringe spacings. These dispersion curves were fitted to a Sellmeier equation which allowed the indices to be determined to ±0.0006. The nonlineardcoefficientsd11,d33,d31, andd13were evaluated at 1000, 1064, and 1300 nm using the Maker fringe technique. The coefficientd11was over 200 times larger than potassium dihydrogen phosphate (KDP)d36. In addition, the nondiagonal coefficientd31was similar to the phase‐matching coefficient in the organic material 3‐acetamido‐4‐dimethylamino‐nitrobenzene (DAN). Critically phase‐matched second harmonic signals were observed at all fundamental wavelengths. There was excellent agreement between the experimentally determined and theoretical phase‐matched incidence angles. Noncritical phase‐matched conditions have been calculated and are reported. Both angle and wavelength noncritical phase matching is possible with this crystal.