The filaments of Leucothrix mucor are able to form true knots under certain cultural conditions. Such structures have apparently not been previously seen in filamentous organisms. As the culture ages, the knots become tighter and eventually the cells in the knot region fuse and form a large bulb. The filament breaks on each side of the bulb, and two shorter filaments and a free bulb are produced. The free bulbs have not been observed to grow into new filaments.