The heat resistance, in milk, of bacteriophages attacking strains of lactic streptococci isolated from cheese starters has been determined. Active phages did not usually survive a temperature of 75°C. for 7½ min. but many phages were not destroyed by heating for 50–60 min. at 65–67°C. and most survived at 70°C. for 10–15 min. The practical implications of these findings in the ‘sterilization’ of milk for starter making have been discussed. Growth in yeast dextrose broth was found to be superior to methylene blue reduction for the demonstration of residual phages after heating.