An in-depth biological and physiochemical evaluation of the utility of the Standardized Aquatic Microcosm (SAM) in the evaluation of degradative organisms has been completed. The bacterium, Alcaligenes denitrificans denitrificans, CR-1, ATCC 53957, has the ability to degrade the riot control material 1,4 dibenz oxazepine (CR) and was used as the test organism. The genus Alcaligenes is a diverse and widespread group found in various types of aquatic and terrestrial environments. In this study the microcosm system used was the SAM as developed by Taub with modifications for the purposes of this study. Four groups of treatments were used: (1) controls; (2) microcosms containing the bacteria A. denitrificans CR-1; (3) microcosms containing the toxicant CR; and (4) microcosms containing CR and A. denitrificans CR-1. Separate probes and sampling equipment were used to sample the A. denitrificans CR-1 containing bacteria and the microcosms without bacteria. The initial concentration of CR, initially 6.5 mg/L, was measured using HPLC. The population density of A. denitrificans CR-1 was followed by plating samples onto a selective CR medium. A. denitrificans CR-1 rapidly colonized the microcosms, even those without CR. CR was rapidly degraded to below the EC₅₀/IC₅₀ levels and detectable concentrations (1 mg/ L) within 18 days. Algal populations recovered. However, the daphnid populations did not recover before the end of the test period. The patterns of nitrogen and oxygen metabolism within the CR dosed microcosms was significantly altered. Species diversity was a poor indicator of toxicant impacts. Replicability in the SAM system was comparable to past experiments containing only toxicant. Airborne cross contamination was a significant problem due to the numerous samples taken during the course of the experiment. However, the SAM has potential as a multispecies test system for following the interactions and impacts of toxicant and degradative bacteria and for the risk assessment of novel organisms.