Cypermethrin (CYP) is a type II pyrethroids insecticide that is extremely toxic to fish even at a very low concentration. It is present in most commercial products some of which contain the synergist piperonyl butoxide (PB). PB increases the lethality of a particular dose of CYP on target species. In this study, the acute toxicity (96 h-LC50 values) of CYP alone, PB, and synergized CYP with PB (CYP:PB) for male guppies, Poecilia reticulata (Peters, 1859) were determined by static bioassay test. First, guppies were exposed to various concentrations (2.00, 4.00, 8.00, 16.00, 32.00, and 64.00 mu g/l (ppb)) of CYP and (1.00, 2.00, 4.00, 8.00, and 16.00 mg/l (ppm)) of PB. The LC50 values of CYP and PB were found to be 3.13 ppb, and 1.71 ppm, respectively. Then, guppies were exposed to various concentrations (0.20, 0.39, 0.78, 1.57, and 3.13 ppb) of CYP:PB at 1:1 ratio and the LC50 value of CYP:PB was found to be 0.76 ppb. The results showed that the mixture of CYP and PB caused an extreme increase in the mortality and a decrease in LC50 value almost four times more than CYP alone, for male guppies.