Coccolith, a calcified scale with species-specific fine structure produced by marine unicellular coccolithophorid algae, consists of calcium carbonate (CaCO3) crystals and a small amount of organic matrices. A novel polysaccharide named coccolith matrix acidic polysaccharide (CMAP) was isolated from the coccolith of a coccolithophorid alga, Pleurochrysis haptonemofera. The structure of CMAP was determined by chemical analysis and NMR spectroscopy including COSY, TOCSY, HMQC, and HMBC to be a polysaccharide composed of the following unit: -> 4) L-iduronic acid (alpha 1 -> 2) meso-tartaric acid (3 -> 1) glyoxylic acid (1 ->. It has four carboxyl groups per a disaccharide unit as observed in another polysaccharide PS-2 characterized previously in Pleurochrysis carterae. CMAP showed a strong inhibitory activity on CaCO3 precipitation. These results suggest that CMAP serves as a regulator in the calcification of the coccolith. (c) 2007 Elsevier Inc. All rights reserved.