This study was an extension of a project conducted by Cech and Hartman (1991) at the Academy of Sciences Laboratories in Ceske Budejouice, Czechoslovakia. During the 1991 study, an unidentified bacterial species dominated a biological phosphorous removal (BPR) system, causing the system to fail. The study indicated that feed composition may have been responsible for the bacteria population shift. The purpose of this study was to examine population shifts of polyphosphorous-removing (PP) bacteria and the unknown bacteria (G bacteria) in a bench-scale sequencing batch reactor (SBR), with emphasis on the effects of feed composition on the populations. The purpose of the study also included characterization and identification of the unknown bacterial species. Six SBRs were operated for three 8-hour cycles per day, over a time period greater than three sludge ages. Each 8-hour cycle included an influent/anaerobic, anoxic, aerobic, and settling/effluent stage. Different feed mixtures of acetate, glucose, and nutrient broth were fed to the reactors. Chemical and physical parameters were tested during each stage of an 8-hour cycle weekly, in addition to microscopic analysis of PP and G bacteria. The results of this study indicated that feed composition did not have as significant an effect on PP and G bacteria as hypothesized. G bacteria did not always dominate a system fed a glucose mixture, nor did PP bacteria always dominate a system fed only acetate. In terms of P-removal, bacteria numbers of either PP or G bacteria were not always indicative of P-removal in the reactor. Instead, the size of the PP cells and the amount of phosphorous inside the cells were better indicators of P-removal. G bacteria did not appear to be a controlling factor in PP population size; the reverse appeared to be true. Temperatures above 25C did have an adverse effect on G bacteria, and a positive effect on PP bacteria. No identification in terms of genus and species could be determined for the G bacteria. / Master of Science