The invention relates to a process for the preparation of N-benzyloxycarbonyl-α-L-aspartyl-L-phenylalanine methyl ester (Z-APM) by high-conversion enzymatic coupling of N-benzyloxycarbonyl-L-aspartic acid (Z-Asp) and L-phenylalanine methyl ester (L-PM) in a reaction mixture comprising an aqueous medium, a neutral protease as enzyme and an alkali metal salt, alkaline earth metal salt or ammonium salt, with formation of a precipitate. The enzymatic coupling of Z-Asp and L-PM is carried out in a continuous or fed-batch process at a pH of from 5.0 to 6.5 and an average charged molar ratio of Z-Asp and L-PM between 1:1 and 2:1 and wherein the actual molar ratio of Z-Asp and L-PM is higher than the average charge molar ratio. Preferably the alkali metal salt, alkaline earth metal salt or ammonium salt is present in an amount of from 3 to 25 %, calculated as % by weight based on the total reaction mixture.