To investigate the effects of imidacloprid on natural stream communities under controlled conditions, experimental streams were dosed with imidacloprid at concentrations ranging from 0.061 to 220 micrograms per liter (µg/L) for a period of 10 days. Natural stream macroinvertebrate communities were obtained from the Cache La Poudre River (Larimer County, CO) by colonizing trays full of large gravel for 75 days. The trays were added to laboratory experimental streams (mesocosms) at the USGS Fort Collins Science Center in Fort Collins, CO. At the end of 10 days, invertebrate abundance metrics were analyzed in relation to measured imidacloprid concentrations in water from the experimental streams. Declines in total abundance and Ephemeroptera, Plecoptera and Tricoptera taxa abundance were observed in the mesocosm communities at concentrations of 1 µg/L and above, and one sensitive species, Baetis tricaudatus, was affected at concentrations as low as 0.060 µg/L.