Meiosis generates haploid gametes from diploid cells and is an almost universal feature of eukaryotic organisms. But little is known about how the snitch from mitotic to meiotic cell cycles is molecularly controlled. In the fission yeast Schizosaccharomyces pombe, inactivation of the protein kinase Pat1(Ran1) upon nutrient deprivation triggers entry into the meiotic cell cycle(1-6). Here we show that the RNA-binding protein Mei2 is a substrate of Pat1 kinase and that dephosphorylation of Mei2 is sufficient to switch cells from the mitotic cell cycle into meiosis. Mei2 is localized mainly in the cytoplasm of proliferating cells but is seen as a single spot dose to the microtubule organizing centre in prophase nuclei during meiosis. Our results, and others from a metazoan(7), emphasize the crucial role of RNA-binding proteins in the initiation and execution of meiosis.