Human processes such as industrial effluents and household activities have negatively affected local bays, oceans, rivers, and streams. Runoff is water that exits into bodies of water that contains pollutants such as chemicals or other products that could be potentially harmful to aquatic organisms and often contains toxins from human activities. A study was initiated to evaluate toxin levels as they relate to Daphnia magna toxicity. Methods/Materials: Three different pollutants commonly found in runoff were tested: nitrates, phosphates, and fipronil (which is a pesticide found in the environment and is used commonly in household activities). Through research, the fact was attained that nitrates and phosphates were most commonly found in the San Francisco Bay. The approximate runoff concentrations of nitrates, phosphates, and fipronil were found from various media. The approximate nitrate concentration is 5.8 x 10-5 M, the approximate phosphate concentration is 1.78 x 10-4 M, and the approximate fipronil concentration is 4.9 x 10-3 ug/L. Next, two 24- and 48-hour preliminary tests were set up for the nitrates and phosphates with Daphnia magna (a stock toxicity test and a preliminary dilution bioassay) as well as a distilled water control test. A final bioassay with duplicates of five test tubes of decreasing dilutions was set up with all learned parameters and controls for nitrates and phosphates. Following this, fipronil was tested in a bioassay with three dilutions (1x the average runoff concentration, 100x the average runoff concentration, and 1/100 x the average runoff concentration). In addition, samples of creek water were used from three locations of the Los Gatos Creek: near Roberts Road, near Lark Avenue, and in Vasona Park (all in Los Gatos, California).Duplicates were tested for each sample. Lastly, a nitrate and phosphate kit was used to test the validity of the stock solution concentrations. Results: The phosphate concentrations were actually a lot smaller than previously believed (0.4 ppb instead of 16200 ppm) from the phosphate test kit. However, the stock concentration at 1.78 x 10-7 M proved a 100% death rate. In addition, the approximate concentration at which half of the daphnia died proved to be 1.78 x 10-8 M. For nitrates, the concentrations were accurate. The death rate for Day 1 proved to be 30%, and for Day 2, 56.5%. The approximate concentration in which half of the Daphnia magna died was: 5.8 x 10-5 M. For fipronil, there was a 100% death rate for Days 1 and 2 and the approximate concentration in which half of the daphnia died was: 4.9 x 10-2 ug/L. The creek water results proved to be inconclusive. Conclusion/Discussion: Fipronil pesticide proved to be the most toxic pollutant to Daphnia magna, while phosphates came in second, and nitrates in third. This is important because this information can lead to insight into prioritizing the removal and prevention of pollutants in runoff water.