A new polymorphic modification (space group Pca2₁, Z = 4) obtained by crystallization from acetonitrile solution has been discovered for 2-adamantylamino-5-nitropyridine (AANP), a prospective NLO material. This new phase is isostructural with the known crystal structure of 2-cyclooctylamino-5-nitropyridine (COANP) and is characterized by the optimal molecular orientation for the highest effective NLO responses in the solid state. On the contrary, AANP crystals obtained from the melt or vapor phase belong to another space group Pna2₁ (Z = 4) and have less optimal molecular orientation for NLO responses. New accurate single crystal X-ray diffraction data including low-temperature data were obtained for both compounds (COANP and two phases of AANP). It was found that at room temperature the structure of COANP is characterized by a crystal disorder related to the flexibility of the cyclooctyl ring, but no new crystal phases were found for this compound. For both compounds molecular mechanics calculations of the molecular conformations have been performed, together with calculations of the crystal packing and crystal morphology. Both linear and quadratic electro-optical effects of COANP and AANP crystal films were examined by an ac modulation method using a longitudinal setup, and the electrooptical coefficient r₅₁ and figures of merit were estimated and compared.