Endopolyphosphatases (Ppn1) from yeast and animal cells hydrolyze inorganic polyphosphate (poly P) chains of many hundreds of phosphate residues into shorter lengths. The limit digest consists predominantly of chains of 60 (P(60)) and 3 (P(3)) P(i) residues. Ppn1 of Saccharomyces cerevisiae, a homodimer of 35-kDa subunits (about 352-aa) is of vacuolar origin and requires the protease activation of a 75-kDa (674-aa) precursor polypeptide. The Ppn1 gene (PPN1) now has been cloned, sequenced, overexpressed, and deleted. That PPN1 encodes Ppn1 was verified by a 25-fold increase in Ppn1 when overexpressed under a GAL promoter and also by several peptide sequences that match exactly with sequences in a yeast genome ORF, the mutation of which abolishes Ppn1 activity. Null mutants in Ppn1 accumulate long-chain poly P and are defective in growth in minimal media. A double mutant of PPN1 and PPX1 (the gene encoding a potent exopolyphosphatase) loses viability rapidly in stationary phase. Whether this loss is a result of the excess of long-chain poly P or to the lack of shorter chains (i.e., poly P(60) and P(3)) is unknown. Overexpression of the processed form of Ppn1 should provide a unique and powerful reagent to analyze poly P when the chain termini are unavailable to the actions of polyPase and poly P kinase.