We identified an allele of Saccharomyces cerevisiae CDC20 that exhibits a spindle-assembly checkpoint defect. Previous studies indicated that loss of CDC20 function caused cell cycle arrest prior to the onset of anaphase. In contrast, CDC20-50 caused inappropriate cell cycle progression through M phase in the absence of mitotic spindle function. This effect of CDC20-50 was dominant over wild type and was eliminated by a second mutation causing loss of function, suggesting that it encodes an overactive form of Cdc20p. Overexpression of CDC20 was found to cause a similar checkpoint defect, causing bypass of the preanaphase arrest produced by either microtubule-depolymerizing compounds or MPS1 overexpression. CDC20 overexpression was also able to overcome the anaphase delay caused by high levels of the anaphase inhibitor Pds1p, but not a mutant form immune to anaphase-promoting complex- (APC-)mediated proteolysis. CDC20 overexpression was unable to promote anaphase in cells deficient in APC function. These findings suggest that Cdc20p is a limiting factor that promotes anaphase entry by antagonizing Pds1p. Cdc20p may promote the APC-dependent proteolytic degradation of Pds1p and other factors that act to inhibit cell cycle progression through mitosis.