PURPOSE: To produce a dipeptide, in high efficiency, in the peptide synthesis with thermolysin using N-substituted phenylalanine or N-substituted aspartic acid and a phenylalanine lower alkyl ester as the substrate, by using the substrate in the form of a specific emulsion.

CONSTITUTION: A dipeptide is produced by reacting an N-substituted phenyl- alanine or an N-substituted aspartic acid with a phenylalanine lower alkyl ester (hereinafter called as Ph-ester). The reaction is carried out by contacting immobilized thermolysin with an O/W type emulsion having a volume ratio (aqueous phase/organic phase) of 1:1 and obtained either by dispersing a mixture of an N-substituted phenylalanine and a water-immiscible organic solvent in water containing Ph-ester, or by dispersing a water-immiscible organic solvent in water containing N-substituted aspartic acid and Ph-ester.

USE: Synthetic reaction reagent of peptide having physiological activity or a precursor of synthetic sweetener.