beta-Glucosidase from Thermus thermophilus has specific hydrolytic activity for the outer glucose at the C-20 position in protopanaxadiol-type ginsenosides without hydrolysis of the inner glucose. The hydrolytic activity of the enzyme for gypenoside XVII was optimal at pH 6.5 and 90 A degrees C, with a half-life of 1 h with 3 g enzyme l(-1) and 4 g gypenoside XVII l(-1). Under the optimized conditions, the enzyme converted the substrate gypenoside XVII to ginsenoside F-2 with a molar yield of 100 % and a productivity of 4 g l(-1) h(-1). The conversion yield and productivity of ginsenoside F-2 are the highest reported thus far among enzymatic transformations.