Rates of dissipation of 6,7-di-hydrodipyrido [l,2-a:2[image],l[image]-c] = pyrazidiinium salt (diquat) and 1,1[image]-dimethyl-4,4[image]-dipyridinium salt (paraquat) in water were studied from 1963 to 1965 at Davis, California. Effects of several environmental factors on dissipation of the herbicides, and herbicidal control of 9 to 14 aquatic weeds and toxic ity to three kinds of fish and an Asiatic clam also were investigated. Small amounts of the herbicides were still detectable in water at the end of 12 days. In reservoirs, diquat applied at 1000 ppbw or less dissipated to 9 ppbw or less. In growth pools, a concentration of 4000 ppbw dissipated to between 600 and 900 ppbw. Dissipation of paraquat in reservoirs was slower than diquat and exhibited unpredictable fluctuations in amounts present. When 250 to 2500 ppbw of paraquat were applied in reservoirs, 0 to 180 ppbw remained after 13 days. In growth pools, 80 to 300 ppbw of paraquat persisted on the 12th day after applications of 1000 to 3000 ppbw.