The refractive indices of structurally well-characterized crystals of the new optically nonlinear organic crystal (S)-3-methyl-5-nitro-N-( I-phenylethyl)-2-pyridinamine have been measured from 480 to 660 nm and fitted to four-parameter Sellmeier equations with an accuracy of +/-3 x 10(-3) to +/-5 x 10(-3). The value of n(y) was accurately verified by the measurement of the external angle of incidence required, at the (101) plane, to observe type I phase matching with a fundamental wavelength of 1064 nm. The phase matching locus has been established and is of Hobden class eight. The values of the n(x), n(y), and n(z) refractive indices at 532 nm were found from the Sellmeier equations to be 1.690, 1.901, and 1.725, respectively, and their relative magnitudes are correlated with the molecular arrangement in the crystal lattice. This material is a member of a series of pyridinamines that can exhibit the phenomenon of frequency-dependent orientation of their dielectric axes. Since this compound crystallizes in an orthorhombic structure, rotation of the dielectric axes is precluded for symmetry reasons.