Constructed wetlands for mitigation of nonpoint agricultural runoff have been assessed for their ability to decrease potential toxicity from associated contaminants. After a simulated runoff event, constructed wetlands positioned in series were used to measure the effects of the organophosphate insecticide diazinon. Water, sediment, and plant samples from five sites were analyzed for diazinon concentrations from 0.5 hours to 26 days; peak concentrations were measured in sediment after 0.5 hours (268.7 microg/kg) and in water and plant tissue after 3 hours (121.71 microg/L and 300.7 microg/kg, respectively). Cholinesterase activity and changes in shell growth were measured from Corbicula fluminea deployed at corresponding sites. Water collected after 9 hours from all wetland sites contained diazinon concentrations sufficient to cause toxicity to Ceriodaphnia dubia, but not to Pimephales promelas. C. dubia survival was decreased in water sampled through 7 days from the site nearest runoff introduction, whereas C. fluminea deployed at this same site experienced 100% mortality after 26 days. Clams from lower sites survived wetland conditions, but growth and ChE activity were significantly decreased lower than that of clams from a control site. C. dubia exposed to water from these sites continued to have decreased survival throughout the 26-day sampling. Sediment sampled from 48 hours through 14 days at the lowest wetland site decreased the laboratory survival of Chironomus dilutus, and sediment from upper sites elicited an effect only on day 26. Although wetland concentrations of aqueous diazinon were decreased lower than toxic thresholds after 26 days, decreased ChE activity in deployed clams provided evidence of residual diazinon effects to deployed organisms.