Mucoid colonial forms of Pseudomonas aeruginosa were obtained by subculture from areas of “sliming” around zones of lysis on phage-typing plates. These mucoid forms were not spontaneous variants, but depended on the presence of phage in the lytic cycle for their initiation and continued existence. The properties of naturally occurring and phage-induced mucoid forms were similar. The mucoid colonial form was observed frequently only in patients with chronic disease of the respiratory tract.