The S-1(B-1(2))-S-0((1)A(1)) electronic transition of m-difluorobenzene cooled in a supersonic free jet has been investigated in detail by use of laser induced fluorescence excitation and dispersed fluorescence spectroscopy. By analysis of over 50 vibronic transitions we have assigned 16 new S-1 vibrational frequencies, and have confirmed seven previous assignments. This advances the total number of known S, fundamental vibrational frequencies to 23 out of the possible 30. The assignments of two SO frequencies have also been somewhat modified. Two different types of symmetry-forbidden transitions were identified : transitions involving one-quantum changes in b(2) modes were found; and transitions involving combinations of b(1) vibrations were observed to be unusually active in the vibronic spectrum, The observation of these forbidden transitions in both the fluorescence excitation and dispersed fluorescence spectra are explored in terms of first- and second-order vibronic coupling.