Meiosis and sporulation in the yeast Saccharomyces cerevisiae requires social communication, mediated by an extracellular factor which is secreted from cells during sporulation and accumulates in a cell density-dependent manner. We show here genetic and biochemical analyses supporting our conclusion that the extracellular factor is bicarbonate acting as an alkali to elevate extracellular pH. Sporulation defects of mdh1 (mitochondrial malate dehydrogenase) mutants and of wild-type cells at low density were rescued extracellularly by addition of bicarbonate or other alkaline solutions to raise medium pH. Addition of bicarbonate (or alkalization of medium) raised steady-state levels of mRNA in respiration-deficient mdh1 mutants and inhibited proliferation of wild-type cells at low density. These results indicate that the two conditions (respiration competency and high cell density), required for meiosis and sporulation, are essential for extracellular accumulation of bicarbonate and resulting alkalization of medium.