The synthesis of L-tyrosine fructosyl ester, from fructose and L-tyrosine methyl ester, was carried out by a transesterification reaction catalyzed by alpha-chymotrypsin in water without cosolvent. The effect of fructose concentration and temperature for the transesterification reaction were determined on both specific activities and product yield. The influence of the presence of fructose has been studied regarding alpha-chymotrypsin and L-tyrosine fructosyl ester stabilities. It appeared that an increase of temperature enhanced enzyme activity but slumped the product yield because of the very weak stability of tyrosine fructosyl ester.