The order-disorder transition (ODT) of semidilute solutions of polystyrene-block-polyisoprene in toluene was studied by small-angle X-ray scattering (SAXS) and dielectric relaxation. Both methods clearly revealed, for the first time for a semidilute solution, a sharp order-disorder transition at the same order-disorder transition temperature. The SAXS studies indicated that the ODT is the thermally induced first-order phase transition from the "non-mean-field disordered state" to an ordered state.