A microfiltration (MF) membrane bioreactor was developed for an efficient production of a recombinant thermostable alpha -glucosidase (rSsGA) from Sulfolobus solfataricus MT-4. The aim of the membrane bioreactor was to improve the control of the concentration of key components in the growth of genetic engineered microorganisms, such as Escherichia coli. The influence of medium composition was studied in relation to cell growth and alpha -glucosidase production. The addition of components such as yeast extract and tryptone resulted in a higher enzyme production. High cell density cultivation of E. coil BL21(DE3) on semidefined medium, exploiting a microfiltration bioreactor, was studied in order to optimize rSsGA production. In addition to medium composition, the inducer employed (either isopropyl beta -D-thiogalactopyranoside or lactose), the induction duration, and the cultivation mode influenced both the final biomass and the enzyme yield. The MF bioreactor allowed a cell concentration of 50 g/L dry weight and a corresponding alpha -glucosidase production of 11,500 U/L. The improvement obtained in the enzyme production combining genetic engineering and the microfiltration strategy was estimated to be 2,000-fold the wild-type strain.