  Multiple scattering, modified fragmentation functions and radiative energy
loss of a heavy quark propagating in a nuclear medium are investigated in
perturbative QCD. Because of the quark mass dependence of the gluon formation
time, the medium size dependence of heavy quark energy loss is found to change
from a linear to a quadratic form when the initial energy and momentum scale
are increased relative to the quark mass. The radiative energy loss is also
significantly suppressed relative to a light quark due to the suppression of
collinear gluon emission by a heavy quark.
