We show here that the state of the p34cdc2-p56cdc13 mitotic B cyclin complex determines whether a fission yeast cell undergoes S phase or mitosis. Mutants defective for p56cdc13 reset to G1 and rereplicate their DNA, while cells completely lacking the p34cdc2-p56cdc13 complex undergo multiple rounds of S phase. In contrast, formation of the p34cdc2-p56cdc13 complex in G1 promotes cells inappropriately into mitosis. We propose that the temporal order of S phase and mitosis is maintained by the presence or absence of the p34cdc2-p56cdc13 complex.