Archaea are expected to be highly repair proficient since they survived the vicious onslaught of radiation damage at the time of their early appearance. The DNA double strand break repairing ability of mesophilic archaea Methanosarcina barkeri (DSM 804) was studied using 7Li, 12C and 16O heavy ions and compared with that of 60Co γ-rays. They can repair double strand breaks and, as in eukaryotes, the nature as well as extent of induction and its subsequent repair were dependent on the linear energy transfer of the radiation source. © 2003 Elsevier Science Ltd. All rights reserved.