Though the use of rotenone is a common procedure in freshwater fishery management, little is known of its effect on zooplankton. Fern Lake, a shallow dystrophic lake in western Washington, was treated with 0.5 p.p.m. of 5 percent rotenone powder in June 1960. Biweekly sampling of the open water, shore edge, and marsh area prior to and for 6 months following treatment provided quantitative and qualitative measurements of zooplankton populations. Open-water species were completely removed, and remained absent for over 3 months. Organisms along the shore edge resisted the effect of rotenone, but eventually disappeared for several weeks. Those species inhabiting the dense weed patches did not all succumb to rotenone. The rare appearance of large numbers of males and usually early ephippial-egg-bearing female Cladocera were observed.