In situelectron irradiation damage has been produced in foils of magnesium of two purities (99.995% and 99.9%) in the 200 kV electron microscope. The concentration of the electron beam-induced point defect clusters in the form- of dislocation loops in the 99.9% magnesium was a factor of ten higher than that in the 99.995% material, indicating that loop nucleation and/or growth were sensitive to specimen purity. The growth kinetics of most of the dislocation loops indicated that the surfaces of the thin foils acted as dominant sinks for the electron-produced point defects. The nature of dislocation loops previously induced in magnesium by neutron irradiation was determined from their behavior under subsequent electron irradiation.