The bitterant denatonium benzoate is being used in automotive coolants to deter human ingestion. The environmental impact of large scale use of bitterants has not been reported. Studies on the biodegradability of denatonium benzoate suggest that the benzoate is primarily responsible for the reported biodegradation. Since the aversive taste is associated with the denatonium ion, denatonium chloride was investigated in the Semi-Continuous Activated Sludge (SCAS) Test and the modified Sturm CO2 Production Test to determine if the denatonium ion would persist in the effluent after biological wastewater treatment. A microbial inhibition test demonstrated that the material is not toxic to the microorganisms in activated sludge between the concentrations of 1 and 150 mg active/L. The denatonium ion was not removed by microbial degradation or by adsorption onto the sludge solids in the SCAS test. The lack of CO2 production corroborated the lack of biodegradation observed in the SCAS test. The intact denatonium ion in the test effluents after treatment in the SCAS and CO2 production tests was confirmed by HPLC.