We have identified the rum1+ gene as a new regulator of the G1-phase of the fission yeast cell cycle. rum1+ determines the cell cycle timing of Start, by maintaining cells in a pre-Start state until they have attained a minimal critical mass. Cells lacking rum1+ are unable to arrest in pre-Start G1 in response to nitrogen starvation and are subsequently sterile. In addition, rum1+ prevents entry into mitosis from pre-Start G1, as shown by the fact that cdc10 mutants in the absence of rum1+ undergo lethal mitosis without entering S-phase.