The formation of point-defect clusters in irradiated magnesium aluminate spinel is sensitive to composition. No dislocation loops develop in stoichiometric materials bombarded with 1 MeV electrons, but pre-existing screw dislocations climb into helices. This is attributed to difficulties in loop nucleation rather than in the generation and diffusion of point defects or their differential absorption at dislocation sinks. The bias for interstitials is calculated to be 1·6% at 1073 K.