The effect of Prometryne, Ametryne, Diuron, and Fluometuron on the photosynthes is of the aquatic species Lemna perpusilla Torr. was investigated using an oxygraph with a Clark-type electrode. All four herbicides produced immediate strong in hibition of photosynthesis when applied to intact plants. Diuron was the strongest inhibitor, causing a 50% reduction of photosynthetic rate at concentrations of 3.5 x 10-7 M. Ametryne and Prometryne were less inhibitory. Fluometuron had the lowest effect on photosynthesis, producing 50% inhibition at 1.6 x 10-5 M. Experiments on long term effect showed that all herbicides tested, except Fluometuron, became severely inhibitory to Lemna at concentrations exceeding 1 x 10-6 M. Concentrations of these herbicides in the environment exceeding 1 x 10-6 M could constitute a risk to this and other similarly sensitive species.