Abstract: The acylase from Xanthomanas citri was used to synthesize cephalexin (CEX) from 7 - amino- 3 - deacetoxy - cephalosporanic acid (7 - ADCA) and D - α - phenylglycine methyl ester (PGM) as a kinetic controlled condensed reaction. However, X. citri enzyme catalyzes not only CEX synthesis but also PGM hydrolysis and CEX hydrolysis. The results on a rational analysis of the reaction mechanisms showed that higher activity and higher ratio of transferase to hy-drolase activity are favorable for use in reverse micelles system. The enhancement effect of CEX synthesis parallel with avoiding hydrolysis of PGM and CEX was found when X. citri enzyme was solubilized in AOT/isooctane reverse micelles. The enzyme activity in reverse micelles is governed by the hydration ratio as bell- shaped dependence.