Toxicity of 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) on the growth of Chlorella vulgaris was investigated in batch reactors. Results revealed that 4-CP did not adversely affect the growth of algae up to 20 mg/L, however higher concentrations inhibited growth appreciably and no growth was detected at 100 mg/L. 4-CP also caused some physiological changes in the algal cells as increasing initial 4-CP concentration caused a linear decrease in chlorophyll a (chl-a) content of the cell. 2,4-DCP up to 20 mg/L did not exert toxic effect on the growth of C vulgaris, rather an induction effect was evident. Unlike a linear decrease with 4-CP, no exact correlation between 2,4-DCP concentration and chl-a content of the cell was observed, but it was certain that the presence of 2,4-DCP caused some physiological changes in the cell of C. vulgaris. No biodegradation of 4-CP and 2,4-DCP was observed over a 30-day incubation. (c) 2007 Elsevier Inc. All rights reserved.