PURPOSE: To obtain the titled compound useful as a sweetener with a simple process in high yield, by using a specific microbial strain a Alcaligenes genus, etc., in an addition reaction of N-fumaryl-L-phenylalanine methyl ester and ammonia.

CONSTITUTION: The objective compound can be produced by adding ammonia (donor) to N-fumaryl-L-phenylalanine methyl ester (FPM) using a microbial strain belonging to Alcaligenes genus, Achromobacter genus, Corynebacterium genus, Escherichia genus, Flavobacterium genus, Micrococcus genus, Xanthomonas genus, Pseudomonas genus, Arthrobacter genus or Serratia genus and capable of adding ammonia to FPM to form L-aspartyl-L-phenylalanine methyl ester. The microbial strain is e.g. E.coli ATCC 11303.