Aluminium nitride polycrystals have been irradiated to fluences up to 5·2 × 1024neutrons m-2(E > 1·0 MeV) in a thermal-fast mixed reactor, the JMTR. Dislocations induced by the neutron irradiation were studied by high-resolution electron microscopy coupled with a multi-slice simulation. Very small interstitial loops, a few nanometres in diameter, lying on the (0001) plane of AIN were observed directly for the specimens irradiated to fluences higher than 2·4 × 1024neutrons m-2, and it was determined that those loops have a Burgers vector of (c/2)[0001]. The dilation of thecaxis by neutron irradiation was greater than that of theaaxis, and this anisotropy of the lattice dilation is ascribed to the formation of interstitial loops lying on the basal plane.