From the cell wall of a strain of Chlorella vulgaris a sugar was isolated after acid hydrolysis and was identified as 4-O-methyl-D-xylose by the following criteria: (i) mass spectroscopy of its alditol acetate revealed characteristic primary fragments with m/e 117 and m/e 261, and, when one deuterium atom was substituted at C-1, with m/e 262 instead of m/e 261; (ii) after demethylation with BCl3, xylose was identified as its parent sugar by chromatographic methods; (iii) L-iditol: NAD 5-oxidoreductase (sorbitol dehydrogenase) catalyzed the oxidation of its alditol, but not of 4-O-methyl-L-xylitol. 4-O-Methyl-D-xylose amounted to approx. 10% of the cell walls' dry weight or 1.6% of the cells' dry weight.