A putative L-rhamnose isomerase (RhaA) from Thermotoga maritima was purified with a specific activity of 55 U/mg by His-Trap affinity chromatography. The native enzyme was estimated as a 46 kDa tetramer by gel filtration chromatography. The half-lives of the enzyme at 75, 80, 85, 90 and 95 degrees C were 773, 347, 187, 118, and 65 h, respectively, indicating that it is the most thermostable of all RhaAs. Under the optimum conditions of pH 8.0, 85 degrees C, and 1 mM Mn2+, RhaA with 100 U enzyme/ml converted 500 L-xylulose/l to 225 g/l L-lyxose after 3 h, and converted 500 L-fructose/l to 175 g/l L-mannose after 5 h.