The low-molecular-weight protein tyrosine phosphatases (PTPase) exist ubiquitously in prokaryotes and eukaryotes and play important roles in the regulation of physiological activities. We report here the expression, purification and characterization of an active and soluble PTPase from Thermus thermophilus HB27 in Escherichia coli. This PTPase has an optimum pH range of 2.8-4.8 when using p-nitrophenyl phosphate as the substrate. The thermal inactivation results indicate a high thermal stability of this enzyme, with the optimum temperature of 75 degrees C for activity. It can be activated by Mn(2+), Mg(2+), Ca(2+), Ba(2+), and Ni(2+), but inhibited by Zn(2+), Cu(2+), Cl(1-), and SO(4)(2-). These results suggest that this heat-resistant PTPase may play important roles in vivo in the adaptation of the microorganism to extreme temperatures and specific nutritional conditions.