A new kind of virus mutation is reported for coli-phage T5. The mutant form is about a thousand-fold more stable than the wild type in low concentrations of salt. In high salt concentrations or in broth the two varieties of phage T5 appear to be equally stable. No difference in properties of the two forms could be found other than their difference in temperature stability as a function of salt concentration. The mutant form is the same as the wild type in rate of adsorption to host cell, rate of reaction with anti-T5 serum, requirement of calcium ion for growth, and in the parameters of the one step growth curve. Single plaque stocks of the heat susceptible form of phage T5 contain about 0.1 per cent of phage particles which are as heat resistant as the mutants but which are unable to transmit this property to their descendants, that is, they are phenotypically heat resistant but genotypically heat susceptible. It is proposed to designate the heat resistant mutant T5 st for stable and the heat susceptible wild type T5 st+.