Effects of diazinon on macroinvertebrates were assessed in three outdoor experimental channels from mid-May to mid-September, 1980. One channel served as a control and two channels as low and high treatments. Three dosing regimes were employed. The low and high treatment channels were continuously dosed for 12 wk to nominal concentrations of 0.3 and 3 micrograms/1, then increased to 6 and 12 micrograms/1 for 4 wk, then the high treatment was increased to 30 micrograms/1 with the low treatment channel returned to ambient. Diazinon concentrations did not reach the intended levels during the latter two dosing regimes. No consistent interchannel differences were observed in total macroinvertebrate abundance or in species diversity induces. Effects were noted at the lowest concentration for amphipods and insects. As diazinon concentrations were increased interchannel community structure changes became more pronounced. Elevated drift, especially amphipods and snails, occurred in the treated channels 4 wk after dosing began and following the increases in concentrations. Total numbers and species of insects emerging from the three channels were not greatly different but lower numbers of mayflies and damselflies emerged from the treated channels.