  Spin relaxation in-plane anisotropy is predicted for heterostructures based
on zinc-blende semiconductors. It is shown that it manifests itself especially
brightly if the two spin relaxation mechanisms (D'yakonov-Perel' and Rashba)
are comparable in efficiency. It is demonstrated that for the quantum well
grown along the [0 0 1] direction, the main axes of spin relaxation rate tensor
are [1 1 0] and [1 -1 0].
