A unique bacteriophage of Aeromonas hydrophila serotype O:34 was isolated, purified, and characterized. The bacterial surface receptor was shown to be the O-antigen polysaccharide component of lipopolysaccharide specific to serotype O:34, which was chemically characterized. The high molecular weight lipopolysaccharide fraction (a fraction enriched in O antigen) was fully able to inactivate bacteriophage PM1. Phage-resistant mutants of A. hydrophila O:34 were isolated and found to be specifically devoid of lipopolysaccharide O antigen. No other cell-surface molecules were involved in phage binding. The host range of bacteriophage PM1 was found to be very narrow, producing plaques only on A. hydrophila strains from serotype O:34.