Protein phosphatase 2C (PP2C) is one of four major classes of protein serine/threonine-phosphatases, which requires Mg2+ for its activity. PP2C activity distributes ubiquitously among eukaryotes and enzymes purified from mammalian tissues were well characterized biochemically, however, very little is known about their biological function. We have been trying to identify cellular function of PP2C by genetic analyses in fission yeast Schizosaccharomyces pombe. So far, three PP2C homologs, ptc1+, ptc2+, and ptc3+, have been cloned and their activity were detected in the S. pombe cell lysates. Experiments using ptc mutants constructed by gene disruption technique have revealed the involvement of PP2C in the heat shock response and possibly osmoregulation in yeast cells.