A thermophilic bacteriophage was isolated from soil. Heat inactivation of this phage, suspended in tryptone starch broth at 65°C and 70°C, was found to be a monomolecular reaction. The phage was more heat stable in tryptone broth than in tris buffer. When the tris buffer was supplemented with calcium or magnesium ions, the survival percentage increased from 0.0 to 18.0 after two hours of heating at 65°C. The addition of sodium or potassium ions to the tris buffer had no significant effect. Equimolar solutions of calcium and magnesium chloride had the same effect on the heat stability of the phage. Maximum stability was attained in 2.5 mM solutions of these salts, and a further increase in the concentration up to 10.0 mM did not increase the percentage of surviving phages.