We found a novel 52 kDa matrix glycoprotein MPP1 in the shell ofCras-sostrea nippona that was unusually acidic and heavily phosphorylated. Deduced from the nucleotide sequence of kb cDNA, which is likely to encode MPP1 with high probability, the primary structure of this protein shows a modular structure characterized by repeat sequences rich in Asp, Ser and Gly.