The microbial populations in the Hoechst-Celanese activated sludge were examined. Heterotrophs, denitrifiers, sulfate-reducers, protozoa and filamentous bacteria were enumerated. Variations in microbial populations were compared with influent and effluent constituent concentrations, and with aeration basin characteristics, such as dissolved oxygen and F/M ratio, to determine whether any microbial type could be used by plant operators to monitor process performance. Results indicated that filamentous bacteria may be useful to plant operators for monitoring process performance because an inverse relationship between filamentous bacteria, food-to-microorganism ratio and sludge volume index was suggested by this study. Protozoa may also be useful for operators, although more data is needed. Microthrix parvicella and Type 0041 were the most common filament types. Filament Type 1701 was most prevalent during a period of low dissolved oxygen. A strong relationship between stalked ciliates and effluent quality was mention in the literature, but was not found in this study. Enumeration methods were evaluated. / Master of Science