Dichlorvos, which is extensively used organophosphate insecticide, was investigated in the present study for acute toxicity. Guppy (Poecilia reticulata) and carp (Cyprinus carpio) were selected for the bioassay experiments. The experiments were repeated 3 times and 180 guppy and 180 carp were used. Behavioral changes of fishes at each dichlorvos concentrations were observed for the individual fish. Lethal concentration of the insecticide on two fish species were determined using the Finney Probit Analysis Statistical Method. The 96-h LC50 values for guppy and carp were estimated as 1.84 mg/L and 2.51 mg/L respectively.