Successful divisions of eukaryotic cells require accurate and coordinated cycles of DNA replication, spindle formation, chromosome segregation, and cytoplasmic cleavage. The Caenorhabditis elegans gene lin-5 is essential for multiple aspects of cell division. Cells in lin-5 null mutants enter mitosis at the normal time and form bipolar spindles, but fail chromosome alignment at the metaphase plate, sister chromatid separation, and cytokinesis. Despite these defects, cells exit from mitosis without delay and progress through subsequent rounds of DNA replication, centrosome duplication, and abortive mitoses. In addition, early embryos that lack lin-5 function show defects in spindle positioning and cleavage plane specification. The lin-5 gene encodes a novel protein with a central coiled-coil domain. This protein localizes to the spindle apparatus in a cell cycle- and microtubule-dependent manner. The LIN-5 protein is located at the centrosomes throughout mitosis, at the kinetochore microtubules in metaphase cells, and at the spindle during meiosis. Our results show that LIN-5 is a novel component of the spindle apparatus required for chromosome and spindle movements, cytoplasmic cleavage, and correct alternation of the S and M phases of the cell cycle.