Life span and number of cell divisions in eukaryotes are limited. The accumulation of stress-associated damage due to ageing may cause irreversible cell cycle arrest, so-called "cellular senescence". Although many genes have been implicated in determining life span, regulatory systems that counteract age-related stress have not yet been clarified. We examined senescence during a stress of Saccharomyces cerevisiae strains carrying disruptions in protein phosphatase (PPase)-encoding genes in order to identify the system counteracting senescence. Among these strains, short telomeres were found in the sit4 disruptant that lacks one form of protein phosphatase 2A (PP2A). Silencing ability in the subtelomeric region was impaired and hyperphosphorylation of Sir3 was also observed in this mutant. The sit4 mutant was found to have altered nucleoli and a life span as short as an sgs1 mutant. These observations suggest that the PP2A pathway regulates life span in yeast.