Supersonic jet excitation and single vibronic level (SVL) dispersed fluorescence spectra for α- and β-methylnaphthalene (S1 state) are presented. Marked mirror symmetry breakdown between the excitation and origin excited fluorescence spectra of the β isomer is observed. The SVL spectra reveal that mode mixing in the S1 state is too extensive to be accounted for by first order treatment of the Duschinsky effect. Vibronically induced mode mixing in the β isomer is suggested as the mechanism which leads to intramolecular vibrational energy redistribution which onsets at a significantly lower excess vibrational energy than in naphthalene. The implications of our findings for other polyatomics are discussed.