The organic crystal (S)-3-methyl-5-nitro-N-(1 -phenylethyl)-2-pyridinamine has been previously identified by the powder technique as a promising nonlinear optical material. Large single crystals (3 x 3 x 2 cm(3)) of this material have been grown by temperature lowering of a seeded supersaturated solution in acetone, Refined refractive indices were obtained from the predicted angles of incidence of the phase-matched second harmonic signals. Second-order nonlinear optical coefficients, d(ij), as measured by the Maker fringe technique, are 11, 9, and 7 pm V-1 for d(14), d(25), and d(36). respectively. The results are compared with calculations based on the oriented gas model.