A water-soluble ester, methyl formate, was detected as a metabolite in the culture medium of methylotrophic yeasts, Methyl formate synthase, which catalyses NAD(+)-dependent dehydrogenation of the hemiacetal adduct of methanol and formaldehyde, catalyses the ester synthesis, The enzyme activity was inducted on a methanol medium and was increased further by the addition of formaldehyde. In the reaction system using intact cells of Pichia methanolica AKU 4262, 135 mM (8.1 g/liter) methyl formate was produced from 2M methanol, This is a new biological process for ester synthesis that couples spontaneous formation of hemiacetal and alcohol dehydrogenase.