Permethrin, a synthetic pyrethroid pesticide and potential toxic pollutant contaminating aquatic ecosystems, was investigated in the present study for acute toxicity. Guppy fish (Poecilia reticulata) were selected for the bioassay experiments. The experiments were repeated 3 times and the 48-h LC50 was determined for the guppies. The static test method of acute toxicity test was used. Water temperature was regulated at 20 +/- I degreesC. In addition, behavioral changes at each permethrin concentration were observed for the individual fish. Data obtained from the permethrin acute toxicity tests were evaluated using the probit analysis statistical method. The 48-h LC50 value for guppy was estimated as 245.7 mug/l. Values in the range of 0.05-97.0 mug/l have been reported for various other fish species. (C) 2003 Elsevier Science Ltd. All rights reserved.