The spindle checkpoint delays anaphase until all chromosomes are properly attached to spindle microtubules. When the spindle checkpoint is activated at unattached kinetochores, the checkpoint proteins BubR1, Bub3 and Mad2 bind and inhibit Cdc20, an activator of the anaphase-promoting complex (APC). Here, we show that Xenopus laevis Cdc20 is phosphorylated at Ser 50, Thr 64, Thr 68 and Thr 79 during mitosis and that mitogen-activated protein kinase (MAPK) contributes to the phosphorylation at Thr 64 or Thr 68. Cdc20 mutants that are phosphorylation-deficient are able to activate the APC in X. laevis egg extracts. However, Cdc20 mutants in which any of the four phosphorylation sites were altered to Ala or Val failed to respond to the spindle checkpoint signal, owing to their reduced affinity for the spindle checkpoint proteins. This study demonstrates that the spindle checkpoint stops anaphase by inhibiting fully-phosphorylated Cdc20. Our results also have implications for the spindle checkpoint silencing mechanism.