Cyhalofob butyl is a synthetic herbist. This herbicide is a systemic herbicide and is used in weed control especially on celtic agriculture. In this study, the acute toxicity (LC50) of this herbist (Chillinger 200 EC 200, cyhalofob butyl) was investigated, which is currently used intensely in the fight against herbists in Thrace region, including Cyhalofob butyl. Acute toxicity (LC50 value) for this herbicide has been investigated in the common mosquito fish (Gambusia holbrooki) in Thrace Region. Lethal Dose 50 experiments were performed in 3 replicates in static test runs (water temperature 27.70 ±0.56 C, water pH 8.88± 0.37, and conductivity 718.25±21.113 µhos). The experiment results obtained for Chillinger 200 ec during the experiments were evaluated by Trimmed Spearman–Karber method. 96-hour LC50 value for the mosquito fish Gambusia holbrooki was calculated as 4.140 ppm.
