Meiosis and sporulation in the yeast Saccharomyces cerevisiae has been classically viewed as an example of unicellular, eukaryotic differentiation that occurs in response to nutritional starvation. We present evidence that S. cerevisiae produces an extracellular factor(s), called meiosis-promoting factor (MEP), that is required, in addition to starvation conditions, for efficient meiosis and sporulation. This factor is secreted and accumulates in a cell density-dependent fashion such that cells at a low density sporulate poorly under conditions in which cells at a high density sporulate efficiently. Conditioned medium from sporulating cells at a high density contains a small anionic molecule that has cytostatic activity and stimulates sporulation of cells at low density under a normal starvation condition. These results indicate that MEP-mediated social communication between cells is required for meiosis and sporulation.