X‐ray diffraction studies and volumetric measurements in nearly perfect crystals of germanium are reported for samples exposed to successive fast‐neutron fluences above 1020neutrons/cm2at approximately 85°C. Volume expansions greater than 7×10−3were found and complementary lattice parameter results indicate the formation of defect regions having an average density which is considerably lower than that of the ordered structure. Anomalous fast‐neutron dose rate effects were observed by varying this parameter from 2.8×1013neutrons/cm2/sec to 3.5×1014neutrons/cm2/sec.