The flocculation of yeast cells by Lactobacillus fermentum was investigated. Some strains of this bacterium were able to flocculate yeast cells but others were not. The presence of calcium ion stimulated flocculation but partial flocculation was found in absence of this ion. The pH range of deflocculation was different from those found in flocculating yeasts. The treatment of bacterial cells with heat and protease destroyed the flocculating ability. The effect of bacterial flocculation of yeast in alcohol fermentation industry is discussed.2211216