A cDNA encoding a new isoenzyme of β-type carbonic anhydrase (CA; EC 4.2.1.1) in the marine diatom Phaeodactylum tricornutum Bohlin has been cloned. The cDNA contained an open reading frame of 819 bp, which encodes a polypeptide of 273 amino acids. This gene, which is designated as ptca2, was found to be highly homologous (83% at the nucleotide level) to the previously isolated intracellular β-CA gene from Phaeodactylum tricornutum (ptca1). Comparison of the deduced amino acid sequence of ptca2 with β-CAs from other sources demonstrated that PtCA2 possesses the completely conserved zinc coordination residues of β-CA. The N-terminus 19 amino acid sequence of PtCA2 was predicted to be an endoplasmic reticulum-targeting signal, suggesting localization of the protein in an organelle or in the periplasmic space. Quantitative analysis of mRNA accumulation of ptca2 using real-time polymerase chain reaction revealed a significant level of mRNA accumulation even under 5% CO2 and a 3.5-fold increase in accumulation upon acclimation of the diatom to air. This indicates that ptca2 belongs to a constitutive class of enzyme that responds only weakly to the ambient CO2 concentration. The sequences of both ptca1 and ptca2 were shown to be grouped into a phylogeny that is composed of mixture of sequences from the eucarya and procarya domains, including sequences from the red alga Porphyridium purpureum, the green alga Coccomyxa, the red mold Neurospora crassa, and the yeast Saccharomyces cerevisiae.Key words: carbonic anhydrase, marine diatom, inorganic carbon concentrating mechanism (CCM), Phaeodactylum tricornutum.