The purpose of this research was to uncover factors responsible for the prolific growth of various filamentous microorganisms in the bulking activated sludge at the Hoechst Celanese wastewater treatment plant. First, futile attempts were made to isolate the filamentous bacterium, Nostocoida limicola II, from activated sludge for purposes of its characterization. Chemostat and batch experiments were also performed in an effort to determine conditions favoring its growth, but the filament’s growth could not be sustained in any of the conditions provided. Second, several CSTRs were operated in which single cationic concentrations were varied to try to isolate the actual effects of the different cations on activated sludge microorganisms. Though the objectives of these CSTR experiments were not accomplished because bulking conditions could not be maintained in the reactors, insight was gained as to possible factors significantly impacting filamentous growth. A full-scale study followed, in which microbial population shifts in the activated sludge at the Hoechst Celanese wastewater treatment plant were tracked with changes in the operating conditions at the plant. The results of this study suggested the existence of relationships between the abundances of certain filaments and DO concentration and/or F/M ratio in the activated sludge environment. To isolate the individual effects of these two factors on activated sludge microorganisms, two modified batch studies were performed. The results of these studies indicated that the growth of most of the filaments present in the Hoechst Celanese activated sludge is primarily affected by F/M ratio, though DO concentration strongly impacts the growth of some. The findings in the DO and F/M experiments were combined with the findings in the full-scale study to estimate DO concentration and/or F/M ratio ranges in which growth of the following activated sludge filamentous organisms may be favored: Haliscomenobacter hydrossis, Microthrix parvicella, Nocardia species, Nostocoida limicola II, and Types 0041, 0581, 1851, and 1863.