DDT and DDD emulsifiable formulations were tested against aquatic invertebrates in small plots in rapids in the St. Lawrence River. Pollution was minimal; less than 0.0044% of the river discharge was treated in each rest.The weakest dosage of DDD, 0.8 p.p.m.-minutes, disabled many Trichoptera and Diptera; the strongest, 12.0 p.p.m.-minutes, did not eliminate any one Order of insects except perhaps Plecoptera. A dosage of 6.0 p.p.m.-minutes was selected for tests on a larger scale against shadfly larvae (mainly Trichoptera). Net-spinning Trichoptera were more sensitive than case-bearers. Of the non-insect organisms, amphipods were relatively sensitive but their populations were rapidly restored by immigration. Oligochaetes and molluscs were least affected.The use of small plots in a large river required special equipment and techniques: a larvicide dispenser (to be described separately), the use of concentrated milk as a dye to aid in the survey of plot boundaries, the transfer of disabled organisms to fresh water to compare recovery rates, and a formula combining data from samples of both disabled and surviving organisms to estimate per cent kill.