Technical Abstract: A biochemical investigation of arabinogalactan proteins (AGPs) in Physcomitrella patens was undertaken with particular emphasis on the glycan chains. Following homogenization and differential centrifugation of moss gametophytes, AGPs were obtained by Arrive phenylglycoside-induced precipitation from the soluble, microsomal membrane, and cell wall fractions. Crossed electrophoresis indicated that each of these three AGP fractions was a mixture of several AGPs. The soluble AGP fraction was selected for further separation by anion-exchange and gel-permeation chromatographies. The latter chromatography indicated molecular masses of 100 kDa and 224 kDa for the two major soluble AGP fractions. The AGPs in both of these fractions contained the abundant (1,3,6)-linked galactopyranosyl residues, terminal arabinofuranosyl residues, and (1,4)-linked glucuronopyranosyl residues that are typical of many angiosperm AGPs. Unexpectedly, however, the moss AGP glycan chains contained about 15 mole percent terminal 3-O-methyl-L-rhamnosyl residues, which have not been found in angiosperm AGPs. This unusual and relatively nonpolar sugar, also called L-acofriose, likely has considerable effects on the overall polarity of Physcomitrella AGPs. A review of the literature indicates that the capacity to synthesize polymers containing 3-O-methyl-L-rhamnosyl residues is present in a variety of bacteria and algae but became less common through the evolution of land plants to the extent that this sugar has been found in only a few species of angiosperms where it occurs as a single residue on steroidal glycosides.