Rough mutants of two strains of Pseudomonas aeruginosa have been isolated by selection for resistance to virulent lipopolysaccharide- specific phages. The rough mutants fell into two classes on the basis of colonial morphology and agglutination by acriflavine and NaCl. The pigment and exoenzyme-synthesizing properties of these derivatives were identical with those of the parental cultures. The rough strains were 8- to 62-fold less pathogenic for the larvae of the wax moth (Galleria mellonella) than the smooth wild-type cells.