The disaccharide trehalose plays a critical role in yeast cell survival during conditions of environmental stress. The vacuole of the yeast Saccharomyces cerevisiae contains an enzyme, acid trehalase (ATH), that is capable of degrading trehalose. Recently, a gene required for ATH activity, ATH1, was cloned and sequenced [Destruelle et al., (1995) Yeast 11, 1015-1025]. The relationship between ATH1 and ATH, however, was not determined. We have purified ATH and shown that it is the ATH1 gene product; peptide sequences from the purified protein correspond to the deduced amino acid sequence of Ath1p. In addition, antiserum to Ath1p specifically recognizes purified ATH.