In the yeast Saccharomyces cerevisiae , Ras regulates adenylate cyclase, which is essential for progression through the G 1 phase of the cell cycle. However, even when the adenosine 3′,5′-monophosphate (cAMP) pathway was bypassed, the double disruption of RAS1 and RAS2 resulted in defects in growth at both low and high temperatures. Furthermore, the simultaneous disruption of RAS1 , RAS2 , and the RAS -related gene RSR1 was lethal at any temperature. The triple-disrupted cells were arrested late in the mitotic (M) phase, which was accompanied by an accumulation of cells with divided chromosomes and sustained histone H1 kinase activity. The lethality of the triple disruption was suppressed by the multicopies of CDC5 , CDC15 , DBF2 , SPO12 , and TEM1 , all of which function in the completion of the M phase. Mammalian ras also suppressed the lethality, which suggests that a similar signaling pathway exists in higher eukaryotes. These results demonstrate that S. cerevisiae Ras functions in the completion of the M phase in a manner independent of the Ras-cAMP pathway.