Lambda cyhalotrin (TEKVANDO 5EC) belongs to a group of chemicals called synthetic pyrethroids and pesticides and potential toxic pollutant contaminating aquatic ecosystems, was investigated in the present study for acute toxicity. Mosquito fish, Gambusia affinis (Baird & Girard, 1853) were selected for the bioassay experiments. The experiments were repeated 3 times and the 96-h LC50 was determined for the fish. The static test method of acute toxicity test was used. Water temperature was regulated at 20 ± 1 °C, water pH 8.31 total conductive was 621 μS . Data obtained from the lambda cyhalotrin acute toxicity tests were evaluated using the probit analysis statistical method. The 96-h LC50 value for mosquito fish was estimated as 1.107 μg/l.