Abstract
SummaryClones ofStreptococcus lactisand ofStr. cremorisresistant to some bacteriophages that could virtually annihilate the parent strains were selected by plating very large numbers of the bacteria in the presence of the phage. Some of the resistant strains were shown to form acid rapidly in milk and to be satisfactory for cheese-making. They were not lysogenic. Adsorption of the phage to which the parent culture had been sensitive still occurred with 3 strains but not with 4 others. Rates of mutation from phage sensitivity to phage resistance were determined for 2 original strains ofStr. lactisand were found to be of the same order as the rates quoted in the literature for other mutations. The mutation rates of phages to forms which could attack the resistant strains were too low to be measured.
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