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Undoped alfa-Al2O3 single crystals with the c axis either perpendicular or parallel to the broad face were implanted with Mg ions. The concentrations of defects induced by implantation were monitored by optical absorption measurements. In the Mg-implanted region, direct current electrical measurements were made between 296 and 445 K. At room temperature, an increase in the electrical conductivity of about 15 orders of magnitude relative to the unimplanted region was observed only in crystals with the c axis perpendicular to the broad face of the samples. The I&-V characteristics reveal an ohmic behavior of the electrical contact on the implanted area. Measurements at different temperatures suggest that the electrical conductivity is thermally activated with an activation energy of about 0.02 eV. We relate the enhancement in conductivity observed in the implanted regions to intrinsic defects created by the implantation, rather than with the implanted Mg ions.