A polymeric fraction (the putative O antigen) has been isolated from the lipopolysaccharide of the type strain of Burkholderia pickettii. The components of the polymer and their molar proportions were: L-rhamnose (3), 2-acetamido-2-deoxy-D-glucose (1), and O-acetyl (1). By means of NMR studies and chemical degradations, the basic repeating-unit of the polymer was identified as a linear tetrasaccharide of the structure shown. The O-acetyl group is probably located at position 2 of the 3-substituted alpha-L-Rha p. Similar polymers constitute O antigens in the related species Burkholderia solanacearum.