Defect clusters produced in beryllium oxide by neutron irradiation have been examined by transmission electron microscopy of thin chips. Specimens studied were given fast neutron dosages ranging from 5 × 10 19 to ≈ 5 × 10 21 nvt at temperatures ranging from pile ambient to 1200° C. Low temperature, low dose irradiation produces almost imperceptibly small dot clusters which enlarge with both increasing irradiation dose and temperature. In specimens irradiated at approximately 650° C, the dot clusters begin to show evidence of alignment parallel to the BeO basal plane and irradiation at temperatures above ≈ 800° C produces resolvable loops in the basal plane which are believed to be planar groups of interstitials or vacancies. Because of the predominant prismatic cleavage of BeO, these loops are usually observed in cross section and appear as lines perpendicular to the crystallographic c-axis. A few instances of loops oriented in prism planes have also been seen in BeO irradiated at high temperatures. Post-irradiation annealing of BeO containing basal plane defects produced a relatively defect-free structure similar to that of the unirradiated state.