A clump-forming bacterial strain was isolated from a sludge community derived from a waste water treatment plant. The Gram-negative bacterium is hydrophobic and forms an extensive capsule while clumping in dilute medium (0.1% bacto peptone). Emulsan, a capsule inhibitor, does not affect the clumping ability of this bacterium. Clumps are not dispersed with high and low pH, detergents or chelators. Bacterial clumping selected by the waste water treatment processes appears to be a complex set of interactions within and between the strains of bacteria. This study reveals some of the complexities. © 1987 Springer-Verlag.