A pectic polysaccharide SDA was obtained from the boiling water extract of mulberry leaves. It consisted of rhamnose, arabinose, xylose, glucose, galactose, and galacturonic acid units in the molar ratio of 5:4:1:2:6:38. Its molecular weight was determined to be 1.5 x 10(4) by high-performance gel filtration chromatography. A combination of linkage analysis, partial acid hydrolysis, ESIMS, (1)H-NMR, and (13)C-NMR spectral analyses revealed its structural features. It was found that SDA possessed an alpha-(1 --> 4)-galacturonan backbone with some insertions of (1 --> 2) Rha residues, with the side chains attached to the O-3 or O-2 position of GalA residues, or the O-4 position of 1,2-linked Rha residues. The side chains contained branched (1 --> 5) linked arabinan, branched (1 --> 3) linked rhamman, linear (1 --> 4) linked xylan, linear (1 --> 4) linked glucan, and linear (1 --> 2) linked galactan. SDA was a new acidic polysaccharide isolated from mulberry leaves for the first time.