Device for crucible-free zone melting a crystalline rod includes a zone-melting chamber having a sidewall formed with a substantially vertical slot, heating means extending through said slot into said chamber and energizable for forming a melting zone in a substantially vertically supported crystalline rod mounted in said chamber, slide means located adjacent said sidewall and carrying said heating means, said slide means and said heating means being slidable together relative to the rod in the direction of the rod axis whereby the melting zone is passed through the rod, and an elastically deformable sealing strip mounted between said slide means and said sidewall of said zone melting chamber and completely surrounding said slot whereby said slot is gastightly sealed, said sealing strip being secured against displacement.