Fumarate hydratase (EC 4.2.1.2) from the extremely thermophilic archaeobacterium Solfolobus solfataricus has been purified to homogeneity by a rapid purification procedure using affinity chromatography and high-performance size-exclusion chromatography, and the enzyme's physical and biochemical properties have been determined. The native enzyme has a molecular mass of 170 kDa and is composed of identical subunits with a molecular mass of 45 kDa, thus indicating a tetrameric structure similar to fumarases isolated from other organisms. The enzyme was active at temperatures ranging from 40 degrees C to 90 degrees C, with a maximum activity at 85 degrees C. The pH optimum for generation of fumarate was found to be pH 8.0. The enzyme showed high stability to denaturation by heat and organic solvents.