Expression of the gene (OsCA1) coding for carbonic anhydrase (CA) in leaves and roots of rice was induced by environmental stresses from salts (NaCl, NaHCO3 and Na2CO3), and osmotic stress (10%, w/v, PEG 6000). CA activity of rice seedlings more than doubled under some of these stresses. Transgenic Arabidopsis over-expressing OsCA1 had a greater salt tolerance at the seedling stage than wild-type plants in 1/2 MS medium with 5 mM NaHCO3, 50 mM NaCl, on 100 mM NaCl. Thus CA expression responds to environmental stresses and is related to stress tolerance in rice.