The ground- and excited-state dipole moments of a zwitterionic benzimidazole-based pyridinium betaine dye [2,4,6-triphenylpyridinium-1-(1H-benzimidazol-2-ide), hereafter referred to as IB1] were determined, The ground-state dipole moment, calculated from measurements of the static electric permittivity of IB1 in 1,4-dioxane, exceeds 13 D units. The charge distribution in the molecule changes significantly upon excitation : the excited-state dipole moment, calculated from the solvatochromic shift of the low-energy UV-vis absorption band, is equal to ca. 3 D. The experimental results are supported by quantum-chemical calculations, which indicate that both the molecular geometry and positions of the energy levels are crucially influenced by solute-solvent interactions. The vectorial part of the second-order hyperpolarizability of IB1 molecules was estimated from the experimental data employing the two-state model : its off-resonance value amounts to ca. -15 x 10(-40) m(4)/V (-3.6 x 10(-30) esu).