Solubilization of beta-galactosidase into AOT/isooctane reverse micelles was achieved by the injection method while holding its activity. The observed activity in reverse micelles was influenced by the water content Wo. Two peaks in the initial activity were observed, at Wo = 10 and 45. Enzyme stability increased with Wo value. The activity and the stability of beta-galactosidase in reverse micelles are interpreted by the solubilization mechanism of proteins, considering micelle size distribution. Selective back-extraction of solubilized beta-galactosidase from the micellar phase into a new aqueous phase was achieved by using 0.1 M KCl aqueous solution with high activity yield. The separation of beta-galactosidase from E. coli cells was also successfully carried out by the present method where the solubilization of the crude extracts into reverse micelles by the injection method was followed by the next selective back-extraction from the reverse micelles. Enzyme was purified four-fold over the initial crude extract with high activity yield.