The effect of biosurfactant on biodegradability of 2,4-dichlorophenol (2,4-DCP) was investigated by activated sludge bioreactor. In 2,4-DCP treatability experiments, acclimated culture was used in the presence of readily degradable substrate (glucose). JBR 425 rhamnolipid was used as biosurfactant. A test reactor with added biosurfactant and control reactor (without biosurfactant) were used in parallel tests. 2,4-DCP removal efficiency for control and test reactor ranged between 97.4-97.7 and 99.7-99.8%, respectively, when the influent concentration of 2,4-DCP ranged between 30 and 100 mg/l. The removal efficiency of 2,4-DCP decreased up to 24.2% and 32.9% in the control and test reactor, respectively, when the concentration of 2,4-DCP was increased from 100 to 150 mg/l. As a result, application of biosurfactant caused some increase in biodegradation rate of 2,4-DCP. In the test reactor, the presence of biosurfactant stimulated bacterial growth and microorganisms were affected less from the toxicity of 2,4-DCP. (c) 2005 Elsevier Ltd. All rights reserved.