1, The mucilage was extracted from Plantago arenaria seeds and precipitated in acidified alcohol as the "free-acid" polysaccharide. 2, The mucilage had an equivalent weight ca. 2450, uronic anhydride 7,2%, anhydropentose 80%. Periodic acid oxidation indicated that some methyl pentose was present and formed part of a relatively stable nucleus. 3, Autohydrolysis of the mucilage yielded L-arabinose. 4, Hydrolysis with 3% oxalic acid gave an insoluble residue (2%), D-xyxose(62%), L-arabinose(17%) ,D-galactose (6%) and an aldobiuronic acid(l3%). 5, Drastic hydrolysis of the aldobiuronic acid yielded D-galacturonic acid and L-rhamnose and oxidation yielded mueie acid and L-rhamnonic acid. On methylation and hydrolysis, 2:3:4-trimethyl-D-galacturonic acid and 3:4-dimethyl-L-rhamnose were obtained. Thus the aldotiuronic acid was identified as 2-gamma-D- galacturonosido-L-rhamnose.