The inhibition and characterization of the a-class carbonic anhydrase (CA, EC 4.2.1.1) from the Halomonas sp. are reported for the first time. The enzyme was purified 91-fold with a yield of 39%, and a specific activity of 600 U/mg proteins was obtained. It has an optimum pH at 7.5, an optimum ionic strength at 20 mM and an optimum temperature at 20 C. The following anions, SCN were moderate inhibitors, whereas other anions showed only weak activities. Our findings indicate that these anions inhibit the Halomonas sp. CA (HmCA) enzyme in a similar manner to other a-CAs from mammals investigated earlier, but the susceptibility to various anions differs significantly between the Halomonas sp. and other organism CAs.