Defect cascades produced by fast neutron irradiation of YBa 2 Cu 3 O 7−δ single crystals were studied by transmission electron microscopy. The visible defects were found to have sizes between 1 and 5 nm. Defect densities were obtained as a function of neutron fluence between 2x10 21 and 8x10 21 m −2 ( E >0.1 MeV) and compared to damage calculations. The measured defect density scales linearly with fluence and amounts to 1x10 22 m −3 at a neutron fluence of 2x10 21 m −2 . The defect stability was studied at room temperature and through annealing to 400°C.