  Exciton spin relaxation is investigated in single epitaxially grown
semiconductor quantum dots in order to test the expected spin relaxation
quenching in this system. We study the polarization anisotropy of the
photoluminescence signal emitted by isolated quantum dots under steady-state or
pulsed non-resonant excitation. We find that the longitudinal exciton spin
relaxation time is strikingly short ($\leq$100 ps) even at low temperature.
This result breaks down the picture of a frozen exciton spin in quantum dots.
