Fenitrothion, an organophosphothionate insecticide (CAS number: 122-14-5) 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 three times and the 96 h LC50 was determined for the guppies. The static test method of acute toxicity test was used. The water temperature was regulated at 23 +/- 1 degrees C. In addition, behavioral changes at each fenitrothion concentration were observed for the individual fish. Data obtained from the fenitrothion acute toxicity tests were evaluated using the probit analysis statistical method. The 96 h LC50 value for guppy was estimated as 3.28 mg l(-1). Values in the range of mu g l(-1) and mg l(-1) have been reported for various other fish species. Copyright (C) 2007 John Wiley & Sons, Ltd.