Telomeres are essential for chromosome stability, but their functions at specific cell-cycle stages are unknown. Telomeres are now shown to have a role in chromosome separation during mitosis. In telomeric DNA mutants of Tetrahymena thermophila, created by expression of a telomerase RNA with an altered template sequence, division of the germline nucleus was severely delayed or blocked in anaphase, The mutant chromatids failed to separate completely at the midzone, becoming stretched io up to twice their normal length. These results suggest a physical block in mutant telomere separation.