Microorganisms capable of lysing filamentous bacteria were isolated from activated sludge by screening for their ability to form plaques on an agar plate seeded with cells of Sphaerotilus natans. The supernatant obtained by culturing of these microorganisms lysed filamentous bacteria in a liquid culture. After purification using column chromatography followed by anion-exchange, gel-filtration and hydroxyapatite chromatography, the lytic substance produced by of B. polymyxa showed the highest lytic activity in all of screened bacteria. However, a specific band representing the substance responsible for the lytic activity could not be observed on SDS-polyacrylamide gel. In addition, the lytic substance could not be rendered inactive by various enzyme denaturing treatments. The substance has properties similar to those of a biosurfactant that is often produced by B. subtilis.