Spectral relaxation of photogenerated excitations and stimulated emission is investigated in an alpha -sexithienyl single crystal. We have performed femtosecond differential transmission spectroscopy, probing the spectral region from 1.85 to 2.6 eV, and time-resolved photoluminescence measurements at low temperature. Stimulated emission with amplification of the probe light which results in a positive differential transmission signal is observed at two spectral positions for polarization parallel to the molecular c-axis. The observed stimulated emission decays with a time constant of 85 ps. Different kinetics of the rise of the stimulated emission and the superimposed excited-state absorption has been observed for excitation with 3.1 eV. This difference vanishes for excitation with 2.3 eV. Stimulated emission proceeds also via trap luminescence.