IT has been found that a recently isolated strain of Acetobacter caused yeasts to die when both organisms were present in a nutrient medium in full screw-capped bottles. In a typical test full McCartney bottles of beer were inoculated with 5 × 105 cells per ml. of Saccharomyces cerevisiae and amounts of a suspension of a washed culture of Acetobacter were added to different bottles to give counts of from 0 to 50 × 105 cells per ml. The numbers of viable yeasts were determined daily by methylene blue staining during incubation at 25°, with the results shown in Fig. 1.