Nickel (II) chloride, an ubiquitous metal salt which occurs in soil, water, air and in the biosphere was investigated in the present study for acute toxicity to adult male guppy fish (Poecilia reticulata) using bioassay experiments. The experiments were repeated three times and LC(50) values were determined for the guppies. The static test method of acute toxicity test was used. Water temperature was regulated at 25 +/- 1 degrees C. In addition, behavioral changes at each nickel (II) chloride concentration were observed for the individual fish. Data obtained from the nickel acute toxicity tests were evaluated using Finney's Probit Analysis statistical method. The 24, 48, 72, and 96 h LC(50) values for guppy were calculated as 182.6, 177.1, 176.7 and 173.1 mg/L, respectively.