A biochemical screening procedure was developed to identify mutants of Arabidopsis thaliana in which the polysaccharide composition of the cell wall was altered. Over 5000 ethyl methanesulfonate-mutagenized plants were analyzed by this method, leading to the identification of 38 mutant lines. One complementation group of mutants was completely deficient in l-fucose, a constituent of pectic and hemicellulosic polysaccharides. These mutant plants were dwarfed in growth habit, and their cell walls were considerably more fragile than normal.