Mucoid strains of Pseudomonas aeruginosa are characterized by an overproduction of the extracellular polysaccharide alginate. When suspended into chlorinated swimming-pool water or drinking water samples, mucoid bacteria revealed enhanced survival compared with isogenic nonmucoid cells. Removal of slime from mucoid bacteria abolished chlorine resistance, addition of purified alginate to washed bacteria again enhanced survival. Thus, alginate-containing slime confers protection on P. aeruginosa against chlorine and may contribute to survival of these bacteria in chlorinated water systems.