Pyrococcus woesei (DSM 3773) beta-galactosidase gene amplified by polymerase chain reaction was cloned into KpnI and HindIII binding sites of pET-30LIC expression plasmid. The obtained pGal2 (6785 bp) transcription vector was then transferred to Escherichia coli B121 (DE3) cells. High identity (99.9%) of DNA sequences suggests that beta-galactosidases from P. woesei and Pyrococcus furiosus are closely related. This enzyme from E. coli transformant is a unique thermostable protein in the cells and can be successfully separated by thermal precipitation of other bacterial proteins at 85 degrees C. The crude beta-galactosidase remaining in the solution comprises about 21% of the total amount of proteins extracted from E. coli cells and has maximal activity at pH 5.4 and temperature of 93 degrees C. Isolated enzyme is active at temperatures up to 110 degrees C and the activity loss after 4 h of incubation at 85 and 93 degrees C did not exceed 11 and 15% of the initial value respectively.