Chlorella VT-1 a pentachlorophenol-tolerant microalga was exposed to a wide range of chlorophenols. The alga showed some tolerance to all the chlorophenols at a concentration of 10 mg/l except for 2,4,5-trichlorophenol, which was toxic at all concentrations tested. The chlorophenols inhibited the initial growth of the alga but after 5-10 days the culture resumed growth at a rate comparable with that of the control. The resumption of growth was not due to adaptation to the chlorophenols but was a feature of the cell concentration. (C) 2003 Elsevier Science Inc. All rights reserved.