The sensitivities of tobacco, tomato, pepper, and cucumber seedlings to an aquatic herbicide, fluridone, were examined in growth chamber and greenhouse studies. New leaves of tobacco seedlings became chlorotic after 6-d exposure to fluridone concentrations of 5 μg/L or greater in hydroponic float culture, and dry weights were significantly reduced at 100 μg/L or greater. When treated only once at the beginning of a 2-wk evaluation period, all crops except cucumber exhibited injury symptoms at 100 μg/L. When grown in sand, pepper and tomato were injured at 25 μg/L; tobacco injury occurred at 10 μg/L. Dry weights of pepper, tobacco, and tomato seedlings decreased significantly at 250, 50, and 250 μg/L, respectively, on potting mix; effects on dry weights were not significant for plants on sand. Cucumber was injured by treatment at 100 μg/L or greater in sand, but there were no effects on dry weights in either sand or potting mix. When treated three times weekly for 2 wk with fluridone, pepper, tobacco, and tomato grown in potting mix were injured by 50, 25, and 100 μg/L; when grown in sand, injury occurred at 10, 5, and 10 μg/L, respectively. Dry weights of pepper, tobacco, and tomato were reduced by 50,50, and 100 μg/L, respectively, in potting mix; effects on dry weights were not significant for plants in sand. Cucumber seedlings were damaged by 250 μg/L or higher on potting mix and 100 μg/L or higher on sand, but there were no effects on dry weights regardless of substrate. Threshold injury levels for plants grown in potting mix to the second true leaf stage and then treated three times weekly were 25 μg/L for pepper and tobacco and 50 μg/L for tomato; dry weights were significantly reduced in pepper at 25 μg/L and in tobacco and tomato at 50 μg/L. Cucumber seedlings were not injured by any treatment in this test.