The wall of Characiosiphon rivularis is 10% of the dry weight of the thallus and cytoplasm-free wall hulls stained positively using histochemical stains for proteins and carbohydrates. A protocol was developed to purify wall material. Purified wall was dissolved in 1 N NaOH after 2 h of boiling with no hydrolytic products detectable. Purified, dissolved wall was: 8% protein and 63% carbohydrate using colorimetric assays; 6% ash; and 23% unidentified material. Amino acid analysis of triftuoroaceticacid (TFA)-hydrolyzed wall protein revealed the following: 17 amino acids were present, the seven hydrophobic amino acids constituted 42%, and 10% of the wall was protein. Using HPLC, the neutral monosaccharide composition of TFA-hydrolyzed wall was 81% glucose, 0.2% galactose, 1.1% arabinose, 6.2% maltose, 1.4% oligosaccharides resistant to TFA hydrolysis and 10.3% sugar alcohols. Via HPLC, carbohydrates and their derivatives accounted for 82% of the dry weight of the purified wall. Alpha-amylase treatment killed living thalli and the walls were ruptured as seen by light microscopy. Alpha-amylase digestion of purified wall released 3.2% of the glucose and 12.6% maltose, the remaining 84.2% being in oligosaccharides.