We describe the phenotypes caused by a cold-sensitive lethal mutation (ndc1-1) that defines the NDC1 gene of yeast. Incubation of ndc1-1 at a nonpermissive temperature causes failure of chromosome separation in mitosis but does not block the cell cycle. This defect results in an asymmetric cell division in which one daughter cell doubles in ploidy and the other inherits no chromosomes. The spindle poles are properly segregated to the two daughter cells. The primary visible defect is that the chromosomes remain associated with only one pole, and are thus delivered to one daughter cell. Meiosis II, but not meiosis I, is sensitive to the ndc1-1 defect, suggesting that NDC1 is required for some feature common to mitosis and meiosis II. ndc1-1 appears to define a new class of cell cycle gene required for the attachment of chromosomes to the spindle pole.