Red algae fibers extracted from red algae (seaweed) showed very high opacity when used in the manufacture of white paper. For a basis weight of 60 g/m(2), over 90% opacity was observed in paper made from red algae fibers without the aid of mineral fillers, whereas papers made from wood fibers had 70-80% opacity. Mercury porosimetry and light reflectance measurements at various wavelengths were used to investigate the origin of such high opacities in red algae fibers. The results revealed that red algae fibers had an extraordinarily high specific surface area on account of their very narrow widths (2-4 mu m) and short lengths (500-800 mu m). This unique property of red algae fibers can be best utilized for manufacturing highly opaque, premium-grade, lightweight printing papers without lowering the paper strength properties.