Aluminum irradiated at approximately 60°C with neutrons has been examined by transmission electron microscopy. Results differ from those reported by other investigators. Small dislocation loops and larger dislocation rings form during an exposure of 3×1018nvt (E>1 MeV) and increase in number with increasing irradiation dose. It is suggested that the small loops are prismatic loops which arise from the clustering and subsequent collapse of vacancies produced by neutron bombardment. Dislocations which encounter prismatic loops frequently form rings about the loops or other obstacles.