Cyfluthrin, a synthetic pyrethroid pesticide contaminating aquatic ecosystems as a potential toxic pollutant, was investigated in the present study for acute toxicity. Tilapia fry (Oreochromis niloticus L. 1758) was selected for the bioassay experiments. The 48 and 72 h LC50 was determined for the tilapia fry. The experiments were repeated three times. The static test method of acute toxicity test was used. In addition, behavioral changes at each cyfluthrin concentration were determined for the individuals. Data obtained from the cyfluthrin acute toxicity tests were evaluated using the Probit Analysis Statistical Method. The 48 and 72 h LC50 value for tilapia fry were estimated as 25.82 mu g/L (95% confidence limits: 20.92-43.26) and 21.07 mu g/L (95% confidence limits: 16.21-30.39), respectively.