Three β-type genes coding for carbonic anhydrase and CA activities from Orychophragmus violaceus L. and Brassica juncea L. leaves in response to NaHCO_3-induced bicarbonate stress were examined. Three full-length cDNA CDS sequences were designated as OvCA1, OvCA3, and OvCA4 in Orychophragmus violaceus, and as BjCA1, BjCA3, and BjCA4 in Brassica juncea; these genes encoding β-CAs were identified and characterized. In particular, OvCA1 and BjCA1 encode two putative chloroplast isoforms. OvCA3 and BjCA3 encode two putative cytosolic isoforms. OvCA4 and BjCA4 encode two putative plasma membrane isoforms. Quantitative real-time RT-PCR analysis revealed that OvCA1 and OvCA4 expressions in Orychophragmus violaceus, BjCA1, and BjCA4 expressions in Brassica juncea changed synchronously with CA activities as bicarbonate stress was intensified. Bicarbonate stress synchronously stimulated OvCA1 and OvCA4 expressions along with CA activities in Orychophragmus violaceus at slight stress level; but it decreased CA activity, BjCA1 and BjCA4 expressions, and stimulated BjCA3 expression in Brassica juncea. Orychoophragmus violaceus could better adapt to slight bicarbonate stress than Brassica juncea due to the former exhibiting higher OvCA3 expression levels and CA activities than the latter. The responses of CA1 and CA4 in Orychophragmus violaceus and CA3 in Brassica juncea to bicarbonate stress partly regulate HCO_3- to water and CO₂ supplied to plants. Diverse CA gene expressions can partially account for different adaptation strategies of the two plant species subjected to different bicarbonate stress levels.