The dislocation density in high-purity (99.9999%) aluminium has been decreased to 10 5 -10 7 m -2 by cyclic annealing. In such a crystal, there are only a few sources for vacancy generation, so that it is expected to take a long time to attain the thermal equilibrium concentration of vacancies. In order to study the generation rate of vacancies, the electrical resistance has been measured accurately at high temperatures. It is found that it takes at least several hours to attain the thermal equilibrium concentration. For comparison, generation pro® les from pre-existing dislocations and interstitial-type dislocation loops are estimated with a di usion-limited model. It is concluded that, even in a highquality aluminium crystal, most vacancies are generated from pre-existing dislocations, and only a small fraction is generated by the growth of interstitial-type dislocation loops. The surface oxidizes at a rate of about an atom layer per an hour and is not e ective for the generation of vacancies.Recently, the degree of perfection of crystals has been improved; so it is interesting to know where vacancies are thermally generated in such high-quality crystals. In high-purity (99.9999%) aluminium, the dislocation density can be decreased to 10 6 -10 7 m -2 by cyclic annealing (Deguchi et al. 1977). In such a crystal, there are few sources for vacancy generation, and so small interstitial-type dislocation loops are formed by a temperature rise to around 300 ë C and they grow by emitting vacancies.