A protein expression system recently developed for the thermophilic crenarchaeon Sulfolobus islandicus was employed to produce recombinant protein for EstA, a thermophilic esterase encoded in the same organism. Large amounts of protein were readily obtained by an affinity protein purification, giving SisEstA. Upon Escherichia coli expression, only the thioredoxin-tagged EstA recombinant protein was soluble. The fusion protein was then purified, and removing the protein tag yielded EcSisEstA. Both forms of the thermophilic EstA enzyme were characterized. We found that SisEstA formed dimer exclusively in solution, whereas EcSisEstA appeared solely as monomer. The former exhibited a stronger resistance to organic solvents than the latter in general, having a much higher temperature optimum (90A degrees C vs. 65A degrees C). More strikingly, SisEstA exhibited a half-life that was more than 32-fold longer than that of EcSisEstA at 90A degrees C. This indicated that thermophilic enzymes yielded from homologous expression should be better biocatalysts than those obtained from mesophilic expression.