The classic model of vibronic coupling in dimers and tetramers is used for calculating the vibronic fine structure of the lowest-energy electronic transition in the sexithiophene crystal. Good semiquantitative agreement with experiment is achieved, lending credence to the applied model. The observed intensity pattern is interpreted in terms of the deviations from the adiabatic approximation in closely spaced electronic states. The intramolecular Herzberg-Teller corrections are of lesser importance, but are the probable cause of the minor discrepancies in the simulated spectra. The results suggest a modified interpretation of the experimental fluorescence spectra.