The chemical and physical properties and phytocidal activity of diquat (1:1'-ethylene- 2 :2'-bipyridylium cation) and paraquat (1 :l'-dimethyl-4 :4'-bipyridylium cation) are discussed. Data are presented on the evaluation of herbicidal activity of the various salts of these 2 chemicals under controlled laboratory conditions, in plastic pools, and in earthen ponds. Research indicates that a majority of common submersed and many emergent aquatic weeds may be killed by concentrations of 0.2 to 0.5 ppm cation of these 2 chemicals. Fish toxicity studies indicate a threshold toxic concentration for diquat in excess of 10 ppm cation, and for paraquat in excess of 5 ppm cation. Two new quaternary bipyridyl chemicals, diquat and paraquat, have been subjected to extensive aquatic herbicide evaluation tests for the past 3 years. These tests have been conducted in the laboratory, in plastic pools, and to a limited extent in ponds, lakes, and drainage canals. The herbicidal activity exhibited under these varied conditions has been spectacular. Concentrations needed for aquatic weed control have not shown any harmful effects upon fish present in such treated waters. Since these are relatively new herbicides, this paper includes general information on the chemicals and their development as aquatic herbicides, laboratory and plastic pool evaluations of herbicidal activity on selected submersed and emergent aquatic plants, toxicity of the various salts to several species of fresh water fish, and results of herbicidal activity in ponds.