Two farm ponds in south Florida were treated with a concentration of 0.5 part per million by weight of 1,1'-ethylene-2,2'-dipyridylium dibromide (diquat) on April 4, and May 21, 1962, to determine the effect this aquatic herbicide may have on the bottom fauna and plankton naturally existing in these two bodies of water. Bottom samples were taken from the ponds with a 6" by 6" Ekman dredge before and after treatment. The organisms were sorted from the debris and counted. Plankton samples were also taken before and after treatment to evaluate the effect of this herbicide on the plankton. The results in pond number one indicated no change in the number of bottom organisms before and after treatment. However, Chironomids failed to appear in the samples for the first 2 weeks after treatment. The second pond failed to show any reduction in numbers of this particular group of organisms. A large increase in the average number of organisms occurred after treatment in pond number two. Most of this increase in numbers came from the tremendous numbers of Chaoborus spp. Plankton samples before and after treatment in pond number one did not show any measurable difference. The volume of plankton per cubic meter in pond number two decreased the third day after treatment. However it increased during the next seven days to give a greater volume of plankton than before treatment. Water samples were taken from pond number two at periodic intervals after treatment to determine the persistence of diquat in the pond water. Duckweed (Lemna minor), being very sensitive to diquat, was used as an indicator plant. Diquat was present in the pond water at a concentration of 0.25 p.p.m.w. after three days, 0.01 p.p.m.w. after eleven days, and no detectable diquat sixteen days after treatment.