A biopolymer flocculant-producing bacterium, strain PY-90, was isolated and considered to belong to Bacillus subtilis. For the production of biopolymer flocculant by strain PY-90, a medium containing 2 to 5% L-glutamic acid as a nitrogen source was suitable. The biopolymer flocculant was a homopolymer composed of glutamic acid residues and was presumed to be poly(gamma-glutamic acid). In kaolin suspension, the highest flocculating activity was attained at the biopolymer flocculant concentration of 20 mg/l. The flocculating activity was increased by the addition of Ca2+, and the optimum concentration of which was about 2 to 8 mM. The flocculating activity was high in an acidic pH range of 3.0 to 5.0, and decreased upon heating at 100 degrees C.