Optical properties of three crystal modifications of 5-t-butyl-2,3-dicyano-6[4-(dimethylamino)styryl]pyrazine having different color appearances were measured and interpreted on the basis of crystal structure analysis. Two of the three modifications exhibited a bathochromic spectral shift both in absorption and fluorescence compared with those in a toluene solution of the dye. The crystal structure analysis revealed that a two-dimensional brick-wall structure is formed in both modifications. In contrast, the third modification showed a hypsochromic absorption shift with a large bathochromic fluorescence shift. The crystal structure of this modification was characterized by a one-dimensional stacked molecular column. The optical characteristics of the three modifications were qualitatively interpreted in terms of excitonic intermolecular interaction on the basis of their crystal structures.