A multipurpose wet stormwater detention pond in Pinellas Park, Florida was studied to determine its effectiveness in reducing the load of selected water-quality constituents commonly found in urban streamflow. Water-quality samples, and data on streamflow and precipitation were collected at the outflow and principal inflow of detention area 3 on Saint Joe Creek. To compare the constituent loads entering and leaving the detention pond, flows and water quality were monitored simultaneously at the inflow and outflow sites for six storms, and were monitored intermittently during periods of base flow. Lodas od 19 selected chemical and physical constituents were determined. Because all the stormwater entering the detention pond was not measured at the inflow site, computed stormwater inflow loads were adjusted to account for loads from the unmonitored areas. The ratio of storm- water volume measured at the outflow site to stormwater volume measured at the inflow site was used to adjust inflow loads for individual storms. Pond efficiencies for selected water- quality constituents for each of the storms were estimated by dividing the difference in outflow and adjusted inflow loads by the adjusted inflow load. Stormwater loads of the major ions (chloride, calcium and bicarbonate) and dissolved solids at the outflow site exceeded loads at the inflow site, partly as a result of mixing with base flow stored within the pond. However, the detention pond was effective in reducing the stormwater load of such urban-runoff contaminants as metals, nutrients, suspended solids, and biochemical and chemical oxygen demand. Estimated median pond efficiencies for reducing constituent loads ranged from 25 to more than 60 percent for metals, 2 to 52 percent for nutrients, 2 to 52 percent for nutrients, 7 to 11 percent for two measurements of suspended solids, and 16 to 49 percent for the oxygen- consuming substances. The reductions of constituent loads in stormwater are probably a result of dilution with pond water (particularly for smaller storms), adsorption, chemical precipitation, settling, biologic uptake, and oxidation. The establishment of aquatic vegetation midway through the study appears to have increased the efficiency of the pond in reducing loads of urban-runoff contaminants in stormwater. The efficiency of the detention pond in reducing base-flow loads was estimated by comparing base-flow loads at the out- flow site prior to and after construction of the pond. Loads of major ions and dissolved solids in base flow were reduced at median efficiencies ranging from 17 to 35 percent. Urban-runoff con- taminants in base flow were generally reduced at higher efficiencies. Median efficiencies ranged from 38 to 82 percent for metals, 19 to 83 percent for nutrients, 34 to 45 percent for suspended solids, and 43 to 65 for the oxygen-consuming substances. The reductions in loads in base flow are probably a result of adsorption, chemical precipitation, biologic uptake, and settling within the pond. These processes were more effective in reducing base-flow loads after the establishment of aquatic vegetation in the pond.