  We compute the leading-order collisional energy loss of a heavy quark
propagating through a quark-gluon plasma in which the quark and gluon
distributions are anisotropic in momentum space. Following the calculation
outlined for QED in an earlier work we indicate the differences encountered in
QCD and their effect on the collisional energy loss results. For a 20 GeV
bottom quark we show that momentum space anisotropies can result in the
collisional heavy quark energy loss varying with the angle of propagation by up
to 50%. For low velocity quarks we show that anisotropies result in energy gain
instead of energy loss with the energy gain focused in such a way as to
accelerate particles along the anisotropy direction thereby reducing the
momentum-space anisotropy. The origin of this negative energy loss is
explicitly identified as being related to the presence of plasma instabilities
in the system.
