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Single crystals of 3 hexagonal metals (Zn, Cd, Mg) were irradiated at 4.5K with 3MeV electrons up to doses of $\sim 10^{19}$ $e^{-}$/cm$^{2}$. The irradiation induced defects were investigated by measurements of the Huang diffuse scattering of X-rays, the lattice parameter change and the electrical resistivity change. Only for Zn a random distribution of single interstitial atoms (S.I.A.) and vacancies was found. The S.I.A. was characterized by a large relaxation volume of (3.6±0.7) atomic volumes and a strong anisotropy of the defect displacement field. Consistent with a dumbbell configuration of the S.I.A. the large displacement was along the c-axis. The contribution of a Frenkel defect to the electrical resistivity change was $\rho_{F}$ = (15.3±5.7) $\mu \Omega$cm/at%. For Cd interstitial agglomerates of <10 interstitials were found. Their size increased with the irradiation dose. This indicated interstitial mobility at the irradiation temperature. For Mg interstitial agglomerates of 2-3 interstitials were observed. It was shown, that these agglomerates were formed by interstitial migration during irradiation, similar to Cd. In contrast to Cd, no dose dependence of their size was found. This unique behaviour was tentatively explained by the existence of a nucleation barrier. During thermal annealing in stage II small interstitial agglomerates of ~10 interstitials were found for all 3 metals. In the case of Zn and Cd (c/a $\approx$ 1.85) they seem to nucleate and grow on the basal plane, in the case of Mg (c/a $\approx$ 1.6) however, on prismatic planes.