The gene encoding beta-D-galactosidase from Pyrococcus woesei was PCR amplified, cloned, expressed in Escherichia coli under the control of an inducible T7 promoter, purified and characterized. The expression system was developed by the construction of recombinant plasmid, based on the high copy number pUET1 vector, giving four times more efficient expression of P. woesei beta-D-galactosidase (20 mg of enzyme from 1 liter of culture) than that obtained from a previously constructed one. The recombinant enzymes were purified in a two-step procedure: double heat-denaturation of E. coli cell proteins and affinity chromatography on p-aminobenzyl 1-thio-beta-D-galactopyranoside-agarose. To achieve efficient purification of P. woesei beta-D-galactosidase by immobilized metal-ion affinity chromatography (IMAC), a His-tag was placed either at the N- or the C-terminal of the coding sequence. The obtained fusion proteins revealed the same specific activity of approximately 5400 U/mg, which was 10 times lower than the wild-type beta-D-galactosidase (51100 U/mg). The activity of P. woesei beta-D-galactosidase was enhanced by thiol compounds, Mg(2+) ions and D-galactose, and was inhibited by heavy metal ions and D-glucose, while Ca(2+) ions had no effect.