Signalling via calcium is probably involved in regulating eukaryotic cell proliferation, but details of its mechanism of action are unknown *RF 1,2*.In Schizosaccharomyces pombe, the onset of mitosis is determined by activation of a complex of the p34cdc2 protein kinase and a cyclin protein that is specific to the G2 phase of the cell cycle. This activation requires dephosphorylation of p34cdc2. Wee1, a tyrosine kinase that inhibits p34cdc2 by phosphorylating it, is needed to determine the length of G2 phase. Here we show that calcium-activated pathways in Saccharomyces cerevisiae control the onset of mitosis by regulating Swe1, a Wee1 homologue. Zds1 (also known as Oss1 and Hst1) [4-7] is important in repressing the transcription of SWE1 in G2 phase. In the presence of high calcium levels, cells lacking Zds1 are delayed in entering mitosis. Calcineurin [8-11] and Mpk1 [12,13] regulate Swe1 activation at the transcriptional and post-translational levels, respectively, and both are required for the calcium-induced delay in G2 phase. These cellular pathways also induce a G2-phase delay in response to hypotonic shock.