Community level interactions were studied in non-axenic sequencing batch reactors (SBRs) being used to treat 2,4-dinitrophenol (DNP). Increasing the influent DNP concentrations from 1 to 10 µg ml(-1) eliminated large predatory organisms such as rotifers and ciliated protozoa from the SBRs. Under steady-state conditions at a DNP concentration of 10 µg ml(-1), supplemental additions of glucose enhanced DNP degradation and led to the establishment of a microbial community consisting of five species of bacteria and a variety of microflagellates. The bacteria and flagellates exhibited oscillating population dynamics in this system, possibly indicating predator-prey interactions between these two groups. Only two of the five bacteria isolated from this system could utilize glucose as a growth substrate, and one of these two species was the only organism that could mineralize DNP in the system. The other three bacteria could grow using metabolic by-products of one of the glucose-utilizing strains (Bacillus cereus) found in the reactors. Supplemental glucose additions increased the average size of bacterial floc particles to 172 µm, compared with 41 µm in SBRs not receiving glucose. It is theorized that the enhanced mineralization of DNP in this non-axenic system was attributable to increased community interactions resulting in increased bacterial flocculation in SBRs receiving supplemental glucose additions.