Photoexcitation dynamics in purr single molecular crystals of alpha-hexathiophene are studied using ultrafast photomodulation spectroscopy in the spectral range from 0.15 to 2.3 eV. Frenkel excitons are uniquely described by transient photoinduced absorption and stimulated emission bands at 1.3 and similar to 2 eV, respectively. The transient spectra are interpreted using exciton states identified by one- and two-photon absorption. Exciton dynamics in pure crystals dramatically differs from that in polycrystalline films, which is attributed to a much lower defect density in single crystals. Intramolecular internal conversion rate is found to be similar to 150 fs for excitation energies up to 3.6 eV.