The orientation dependence of the backscattering yield (channeling) of 1.0 MeV protons was used to investigate fast neutron-induced lattice damage in silicon and germanium single crystals. The measurements were made at room temperature and for protons incident in ⟨110⟩ crystal directions. The reactor irradiations were made, using a fast neutron flux Φ =3.3 × 10l2n/cm2-sec (E>0.1 MeV) at 60 °C, to total exposures (Φt) between 2.1 × 1017n/cm2and 3.4 × 1019n/cm2. The percent of lattice atoms displaced was not linear with neutron exposure. The displacements rose rapidly with exposure initially, then assumed a very slow rise having a dependence on neutron exposure of (Φt)0.1. This appears to be some modification of an exponential saturation mechanism. It agrees qualitatively with a thermal annealing mechanism, which predicts a flux dependent (i.e., rate of defect introduction dependent) saturation value occurring even for room temperature irradiations.