Best Management Practices (BMPs) have been shown to be effective in reducing runoff and pollutants from urban areas and thus provide a mechanism to improve downstream water quality. Currently, BMP performance regarding water quality improvement is assessed through measuring each individual contaminant of concern at different locations throughout the BMP to quantify the magnitude of loss of each contaminant. Cumulative pollutant removal from these sampling locations is often used to determine the BMP water quality performance. The research presented herein examines the ability of using toxicity (Microtox toxicity analysis) to assess the overall improvement of water quality in a BMP. Two BMPs will be discussed in this analysis; a rain garden used to treat parking lot runoff and a subsurface catchment (bioinfiltration trench) collecting runoff from a parking garage. Both sites were monitored during storm events for a variety of water quality parameters (i.e. copper, lead, chromium, TSS) and compared to concurrent Microtox toxicity analysis from each storm. Sampling occurred at influent, surface water, and effluent (overflow) locations. Pore water samples were also collected (via lysimeters) at various depths below the two BMPs. Results show a decrease in toxicity as the water passed through the BMPs (although the extent of toxicity reduction varied for each BMP). Therefore, the Microtox toxicity assay can be used to assess water quality improvement resulting from BMP treatment. These BMPs removed a variety of metals as well as other water quality pollutants. However, toxicity reduction could not be attributed to the removal of any one particular contaminant.