Deltamethrin, a synthetic pyrethroid insecticide and contaminating aquatic ecosystems, was investigated in the present study for acute toxicity. The aim of this study was to evaluate LC50 values of deltamethrin on Nile tilapia (Oreochromis niloticus L. 1758) larvae and fry after acute exposure to deltamethrin. The 48 h LC50 values for Nile tilapia larvae and fry were estimated as 1.17 and 1.70 mu g/L, respectively. The bioassay experiments were repeated 3 times using the static test method. Data were evaluated using the U. S. E. P. A. LC50 computer program based on Finney's Probit Analysis Method. In addition, behavioral changes at each group for deltamethrin concentration were determined. All larvae and fry exposed to deltamethrin showed behavioral effects and the extent of behavioral changes increased with increased deltamethrin concentration. The results are significant for reporting deltamethrin highly toxic to fish early life stages under acute exposure.