;Bartlett' pear (Pyrus communis) fruits were picked at the mature, green stage and ripened at 20 C. Fruits at different stages of ripeness (based on flesh firmness) were homogenized, and the sugar and uronic acid contents of cell wall and soluble polysaccharides were determined. Substantial amounts of galacturonic acid and arabinose were lost from the wall fraction as the fruit ripened. Most of this cell wall material was recovered, in an 80% (volume/volume) ethanol-insoluble form, from the soluble fraction of tissue homogenates. Structural analysis of ethanol-precipitable material indicates that it is an acidic (pectic) polymer-bearing side groups containing variously-linked arabinosyl residues.