A rapid ion-exchange method has been used to purify the alginate from the extracellular material of mucoid strains of Pseudomonas aeruginosa isolated from the lungs of cystic fibrosis patients. The structure has been investigated by chemical analysis, infrared spectroscopy, paper chromatography, and gas-liquid chromatography. The alginates contain mainly random or poly(D-mannuronic acid) block structures, and are highly acetylated. The relative viscosity is not correlated with the ratio of D-mannuronic acid to L-guluronic acid residues, or the degree of acetylation. The chemical/physical properties of the alginate from P. aeruginosa are considered in the context of the growth of the organism in the lung.