Barnacle attachment to various foreign materials in water is guided by an extracellular multiprotein complex. A 19 kDa cement protein was purified from the Megabalanus rosa cement, and its cDNA was cloned and sequenced. The gene was expressed only in the basal portion of the animal, where the histologically identified cement gland is located. The sequence of the protein showed no homology to other known proteins in the databases, indicating that it is a novel protein. Agreement between the molecular mass determined by MS and the molecular weight estimated from the cDNA indicated that the protein bears no post-translational modifications. The bacterial recombinant was prepared in soluble form under physiologic conditions, and was demonstrated to have underwater irreversible adsorption activity to a variety of surface materials, including positively charged, negatively charged and hydrophobic ones. Thus, the function of the protein was suggested to be coupling to foreign material surfaces during underwater attachment. Homologous genes were isolated from Balanus albicostatus and B. improvisus, and their amino acid compositions showed strong resemblance to that of M. rosa, with six amino acids, Ser, Thr, Ala, Gly, Val and Lys, comprising 66–70% of the total, suggesting that such a biased amino acid composition may be important for the function of this protein.