Field and greenhouse tests were conducted from 1974 to 1976 to determine the most effective rate and number of applications of bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4)3H-one 2,2-dioxide] for the control of six morningglory (Ipomoea) species. Growth chamber studies were conducted to determine photosynthesisrespiration equilibrium points (PRE-points) on two morningglory species. These units were used to measure the duration of the phytotoxic effect of bentazon on these morningglories from both single and repeat applications. Declining PRE-points indicated declining bentazon toxic activity within the plant. A treatment of two applications of bentazon at 0.84 kg/ha applied at a 4-day interval beginning with 2-week-old morningglory plants was optimal for maintaining a high PRE-point level. In field and greenhouse studies the greatest toxicity, regardless of morningglory species, occurred with bentazon at 0.84 or 1.12 kg/ha applied at 14 days and repeated at 28 days after emergence and 0.56 kg/ha applied three times with the first application made 14 days after emergence. Differences in susceptibility of the individual species indicated the importance of proper species identification before bentazon applications.