Acute toxicity testing of contaminants, continually released into the aquatic ecosystems from industrial and residential areas and representing a potential risk to the aquatic biota is important. Mercury is a highly toxic metal to which, due to its wide usage in agricultural, industrial, medical and other fields, exposure cannot be avoided.The purpose of this study was to examine the acute toxicity of mercuric chloride (HgCl2) on a freshwater species of fish, Capotea fusca,according to the static testfor calculating LC50(lethality concentration for 50%).For this purpose,fish were exposed to mercuric chloride and were not fed for approximately 96 h by adding no food. Ten different concentrations in three replicates were chosen. For each treatment, 10 fish specimens were used. The solutions were prepared by dissolving mercuric chloride (Merck) in distilled water. Results represented that mortality decreased as exposure time increased, so that most of the mortality occurred during the first 24 h. Finally, the Conclusion showed that the LC50value at 72 h and 96 h were 0.539 mg/L.