Strains of halophilic rods, presumably halobacteria, produce substances that inhibit the growth of other halobacteria but not halococci or moderate halophiles. The spectrum of action of these substances, called "halocins," was determined on 39 halobacteria or presumed halobacteria. The properties of halocin H4, produced by strain R-4, were studied usingHalobacterium halobiumas an indicator organism. Halocin H4 is adsorbed by the indicator organism and causes both death and lysis. It is a high molecular weight substance(s), destroyed by heat or exposure to low salt concentrations or treatment with a protease. Experiments are presented to show that it is not a virus.