Together with the Rad50 ATPase, the Mre11 nuclease forms an evolutionarily conserved protein complex that plays a central role in the repair of DNA double-strand breaks (DSBs). Mre11-Rad50 detects and processes DNA ends, and has functions in the tethering as well as the signalling of DSBs. The Mre11 dimer can bind one or two DNA ends or hairpins, and processes DNA endonucleolytically as well as exonucleolytically in the 3'-to-5' direction. Here, the crystal structure of the Mre11 catalytic domain dimer from Chaetomium thermophilum (CtMre11(CD)) is reported. CtMre11(CD) crystals diffracted to 2.8 Å resolution and revealed previously undefined features within the dimer interface, in particular fully ordered eukaryote-specific insertion loops that considerably expand the dimer interface. Furthermore, comparison with other eukaryotic Mre11 structures reveals differences in the conformations of the dimer and the capping domain. In summary, the results reported here provide new insights into the architecture of the eukaryotic Mre11 dimer.