The effect of chemical residues of clomazone on photosynthetic processes has been studiedby using several low concentrations of the herbicide (0, 1, 0.1, 0.01, 0.001, 0.0001 and 0.00001) µM and seedlings of two varieties of tobacco (Nicotiana tabacum L. cv. Virginie vk51, Nicotiana tabacum, L. cv. Xanthi). The content of photosynthetic pigments, the parameters of the chlorophyll-a fluorescence and the JIP-test were performed on an adult leaf (AL) and a young leaf (YL), that gave a complementary designto know the action's mode of clomazone on the plant physiological processes. Clomazone reducedthe total chlorophyll (a+b), carotenoids pigments (reduction in size antenna pigments judged by an increase in the chlorophyll a/b ratio) in young leaves more than adults leaves. The maximal photochemical efficiency (Fv/Fm) of photosystem II (PSII) decreased significantly in youngleavescompared to adult leaves and in (Virginie) variety than (Xanthi) variety. Among the parameters calculated of the JIP-test most affected by the treatment, PIabs, 1-VJ, ABS/RC, DI0/RC, TR0/RC, ET0/RC, ET0/ABS, which indicated acomparable effects of clomazone(1μM, 0.1µM, 0.01µM) between the two types of leaves and the varieties used. More, the results showed that the concentration ( 1μM) was the most effective amongthe other low concentrations used and the (Virginie) variety ismore sensitive than the (Xanthi)variety. We conclude that clomazone has probably two combined functions (physiological, toxic) judged by the different behavior of both types of leaves in the presence of the herbicide.