Sediment samples were collected from 24 stormwater retention and detention basins that drain land used for residential, commercial, and industrial purposes in Maricopa County and were analyzed to determine the chemistry and toxicity of pollutants associated with urban stormwater runoff. Samples were collected between January and November 1994. Summary statistics are presented for pH and soil moisture, concentrations of selected inorganic and organic constituents, and concentrations of organochlorine pesticides in sediments associated with each type of land use. Acute toxicity tests were done on sediment samples using the amphipod Hyalella azteca. Survival rates ranged from 0 (zero) to 95 percent. The results of a comparative time-series analysis on samples from two residential sites collected during a 5-month period also are presented. Background concentrations of inorganic constituents in sediments were determined for six basins, and discrete samples were collected to characterize the spatial variability of constituent concentrations in one residential basin. The effect of sieving on sediment toxicity was determined by testing whole and sieved samples. Survival rates ranged from 0 (zero) to 42 percent for sieved samples and 14 to 75 percent for unsieved samples.