Effect of the deltamethrin, which is extensively used synthetic pyrethroid, was investigated on the lipase activity of Poecilia reticulate in the present study. One control and five experimental groups were exposed to different concentrations of deltamethrin. Three experiments, including 60 guppies each, were conducted. Titration method was used in order to determine the lipase activity. Lipase activity level in control group was 5 U/min, while it was 2.5 U/min in the highest concentration of deltamethrin exposed fish. These results revealed that deltamethrin inhibited the lipase activity which may have a negative effect on nutrition and physiological condition of fish.