Since the inception of the activated sludge process, operating problems have been associated with the growth of excessive numbers of filamentous organisms in the sludge. Since separation of the activated sludge from the effluent by sedimentation is absolutely essential for obtaining proper operation of the process, anything which interfers with the settling of the sludge produces a serious operating problem. Any problem with separation of activated sludge in the secondary settling tank, particularly if large concentrations of sludge solids are being lost into the effluent, may be called 'bulking.' Most of the descriptions of bulking ascribe its cause to an overgrowth of a particular filamentous organism, Sphaerotilus natans. The objective of the research described was to provide data which will serve as a basis for quantitative description of some of the phenomena which produce an activated sludge which separated poorly, to determine which filamentous organisms are associated with which phenomena, and to identify environmental factors in the process which could cause the occurrence of each phenomenon.