The spindle checkpoint prevents cell cycle progression in cells that have mitotic spindle defects. Although several spindle defects activate the spindle checkpoint, the exact nature of the primary signal is unknown. We have found that the budding yeast member of the Aurora protein kinase family, Ipl1p, is required to maintain a subset of spindle checkpoint arrests. Ipl1p is required to maintain the spindle checkpoint that is induced by overexpression of the protein kinase Mps1. Inactivating Ipl1p allows cells overexpressing Mps1p to escape from mitosis and segregate their chromosomes normally. Therefore, the requirement for Ipl1p in the spindle checkpoint is not a consequence of kinetochore and/or spindle defects. The requirement for Ipl1p distinguishes two different activators of the spindle checkpoint: Ipl1p function is required for the delay triggered by chromosomes whose kinetochores are not under tension, but is not required for arrest induced by spindle depolymerization. Ipl1p localizes at or near kinetochores during mitosis, and we propose that Ipl1p is required to monitor tension at the kinetochore. The accurate propagation of genetic information depends on faithful chromosome segregation. Accurate chromosome segregation depends on the precise coordination of events in the chromosome cycle. When chromosomes replicate during S phase, linkage between the sister chromatids (cohesion) is established and must be maintained while chromosomes condense and align on the mitotic spindle. Chromosomes attach to the mitotic spindle by their kinetochores, specialized protein structures that are assembled on centromeric DNA sequences. Once all the chromosomes are correctly aligned on the mitotic spindle, sister chromatid cohesion must dissolve promptly at anaphase to allow the sister chromatids to segregate rapidly to opposite poles of the mitotic spindle. Defects in any of these steps can result in aneuploidy, a hallmark of tumor cells and some birth defects (Lengauer et al. 1997.The spindle checkpoint prevents cells from separating their sister chromatids until chromosome alignment is complete. The conserved components of the checkpoint include the Mad (Mad1-Mad3) proteins, Bub1 and Bub3, Mps1 (a protein kinase), and Cdc55 (Hoyt et al. 1991;Li and Murray 1991;Minshull et al. 1996;Weiss and Winey 1996;Wang and Burke 1997). A separate control, the Bub2-dependent checkpoint, monitors a second aspect of chromosome segregation, the delivery of DNA or a spindle pole body into the daughter cell (Alexandru et al. 1999;Fesquet et al. 1999;Fraschini et al. 1999;Li 1999). Spindle checkpoint defects are associated with genetic instability, and some human cancers contain mutant spindle checkpoint genes (Cahill et al. 1998;Takahashi et al. 1999) The spindle checkpoint monitors the interaction between kinetochores and microtubules. Spindle checkpoint proteins localize to kinetochores (Chen et al. 1996;Taylor and McKeon 1997;Bernard et al. 1998), and all known kinetochore and spindle defects that activate the checkpoint affect the i...