alpha-Galactosidase activity in Rhodothermus marinus is dependent on the composition of the growth media. A maximum of 46 mu U g(-1) cell dry weight was obtained using minimal medium with galactooligo- or polysaccharides as single carbon source. An enzyme hydrolysing both high and low molecular weight galacto-saccharides was partly purified from the cell fractions. The molecular weight was 200 kDa (native) and 50 kDa (monomer). It was optimally active at 85 degrees C, with a half-life of 2 h at 75 degrees C, and had a broad pH range (4-8).