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The emergent hydrophyte Iris pseudacorus was constantly exposed over a 35-day period to atrazine in the laboratory. It could survive at an atrazine level up to 32 mg/L. Its relative growth rates were inhibited significantly when exposure dosage reached at or exceeded 2 mg/L (p < 0.05). No observed effect concentration and lowest observed effect concentration for growth were 1 and 2 mg/L, respectively. Chlorophyll a and b contents of the plant in all treatment groups were affected significantly, and chlorophyll a/b ratios of all atrazine treatment levels were pronouncedly higher than those of the control within 5 days of exposure (p < 0.05), but thereafter recovered to the level of the control. Differences of photosynthetic efficiency were significant between all atrazine treatments and the control; except for 1 mg/L on day 1 and 5, and 2 mg/L on day 1. I. pseudacorus did not show phytotoxicity symptoms after 35 days exposure to atrazine below 2 mg/L level, but photosynthetic efficiency had begun to decline.