Time evolution of the coherence properties of excitons in J-aggregates of an organic molecule is studied by femtosecond transient absorption and three-pulse stimulated photon echo peak shifts. The exciton energy correlation function decays to zero by 120 fs single exponential, manifesting an ultrafast rupture of the coherent wave packet motion and the absence of inhomogeneity beyond the 100 fs time scale. Insensitivity of the correlation function to the isotopic substitution of the solvent suggests that the phonon modes of the molecular chain are responsible for the ultrafast dynamics.