A flocculent aggregate produced by Lactococcus lactis subsp. lactis in broths containing Tween 80, including MRS broth, had a microscopic structure of intertwined thread-like filaments. The filamentous structure was not elongated bacterial cells, but consisted of an organic solvent-soluble portion and an insoluble solid. L. lactis subsp. lactis grown at $25^{circ}C$ for 15 days in tryptic soy broth with 0.1% Tween 80 and 1.0% malt extract produced 13 mg/l of flocculent aggregate, which contained 0.84 g/g of organic solvent-soluble component. The organic solvent-soluble part was identified as 10-hydroxyoctadecanoic acid.