Abstract: Abstract: Acute toxicity of mercury-II chloride (HgCl2), one of most toxic pollutants for aquatic ecosystems, in chub (Alburnoides bipunctatus) and rainbow trout (Oncorhynchus mykiss) was investigated in the present study. Actual concentration of HgCl2 was ranged from 0.10 to 8.00 mg L-1. Concentrations of HgCl2 that killed 50% of the chub and rainbow trout within 96 h (96 h LC50) were estimated as 0.205 and 0.814 mg L-1, respectively. Acute toxicity tests were evaluated by the Finney’s Probit Analysis. Chub (Alburnoides bipunctatus) was appeared to be more sensitive than trouts (Oncorhynchus mykiss) to HgCl2.