Production of a thick exopolysaccharide coat (alginate) by mucoid strains of Pseudomonas aeruginosa has been shown to contribute to the pathogenicity and persistence of these bacteria in the lungs of patients with cystic fibrosis. Previous studies have shown that some mucoid P. aeruginosa strains produce an enzyme(s) capable of degrading this alginate coat. In this study, an alginate lyase from mucoid P. aeruginosa strain WcM#2 was isolated and characterized. Lyase production was enhanced by the addition of 0.2-0.3 M NaCl to the growth media. The lyase was eluted from an alginate-Sepharose affinity column with 0.5 M NaCl, which can serve as a simple one-step purification protocol for obtaining semi-pure functional alginate lyase. Fractionation of the enzyme preparation on a Sephadex G-75 sizing column showed that the enzyme has an apparent molecular weight of 40,000, whereas sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) suggested a molecular weight of approximately 43,000. The affinity-purified enzyme had a pH optimum of 9.0, its activity was enhanced in the presence of 0.3 M NaCl, and it showed substrate specificity for polymannuronic acid blocks. These results demonstrate the presence of a mannuronan-specific alginate lyase in P. aeruginosa that differs in several respects from previous reports of P. aeruginosa alginate lyases.