An extracellular acetyl esterase (EC 18.104.22.168) from Candida guilliermondii NRRL Y-17257 was purified to homogeneity by acetone precipitation and QAE sepharose anion-exchange chromatography. The enzyme was a monomer with an apparent molecular weight of 67 kDa and a pI of 7.6. It had maximum activity at pH 7.5 and at 50-60°C. It was relatively stable over a pH range of 5.8-8.0 and exhibited thermal stability up to 60°C. The K(m) and V(max) values on α-naphthylacetate were 2.63 mM and 213.3 μmol α-naphthol min-1 mg-1 protein, respectively.