Acute toxicities of five heavy metal ions on fertilzed eggs of killifish(Oryzias latipes) were measured in terms of median lethal concentration(LC50) and toxicity curve. The LC50 of 24, 48 and 96 hours were 24.00, 19,80 and 19.80 ㎎/ℓ for nickel; 8.30, 6.30 and 4.5㎎/ℓ mercury ; 6.75, 4.20 and 3.50 ㎎/ℓ for cadmium ; 0.68, 0.56 and 0.56 ㎎/ℓ for zinc ; 0.067, 0.065 and 0.065㎎/ℓ for copper respectively. The toxicity curves derived by polotting LC 50 values against the period of exposure, and the results indicate that effect of copper was most toxic followed by zinc, cadmium and mercury, and the effect of nickel was the least.