We present a novel organic crystal, 5-(methylthio)-thiophenecarboxaldehyde-4-nitrophenylhydrazone, with a large effective phase-matchable nonlinear optical coefficient,deff=29±3 pm/V at &lgr;=1318 nm. Single crystals of dimensions up to 25×5×5mm3were grown. The large off-diagonal nonlinear optical coefficients can be explained with the two-dimensional geometry of the chromophores in the crystal lattice. Our crystal is ideally suited for optic parametric oscillation in the wavelength range from 1000 to 2500 nm. ©1997 American Institute of Physics.