A high temperature stage fitted to a Siemens Elmiskop 1 electron microscope has been used to study directly the annealing of prismatic dislocation loops in thin foils prepared from quenched and aged bulk specimens of aluminium. In the temperature range 170-200°c the loops are observed to shrink and eventually to disappear. Observations on dislocations other than loops also present in the thin foils show that they become smoother while the loops are disappearing. The results are interpreted in terras of climb of dislocations by vacancy emission, and in the case of loops in thin foils, where vacancy supersaturation effects are thought to be negligible, the climb rate is governed by the line tension and curvature of the loops. The radiusrof a loop as a function of timetis given by the theoretical expressionr=r0[1 - (t/τ)]1/2 where τ is the lifetime of a loop of initial radiusr0.The temperature dependence is primarily τ = τ0exp (E/k T) where E is the activation energy of self diffusion. Comparison with experiment gives estimates of the activation energy in the range 1· 2–1 · 3ev.