Acute toxicity of hydrogen sulfide to larval stage of blue crab, Portunus trituberculatus was measured using a continous flow system. Survival rates of crab larvae to oxygen deficient water (DO≦3.4 mgℓ-1) and oxygen deficient water (3.4 mgO2ℓ-1 and 2.6 mgO2ℓ-1) with two different levels of hydrogen sulfide (10 μ-gH2Sℓ-1 and 5 μgH2Sℓ-1)were also determined. Survival rate of crab larvae exposed to hydrogen sulfide for 48 hours decreased at H2S≧5.2μgℓ-1, H2S≧9.5μgℓ-1, H2S≧19.7μgℓ-1 levels to zoea, megalopa and crab stage, respectively. Larval survival of zoea, megalopa and crab stage under low oxygen was significantly affected by DO≦3.4 mgℓ-1, DO≦2.6 mgℓ-1, 1.5 mgℓ-1 levels, respectively. The combined effects of low dissolved oxygen and hydrogen sulfide to the blue crab were stronger than each single effect. Namely, survival rate lowered under the combined exposure to≦3.4mgO2ℓ-1+≧5μgH2Sℓ-1, ≦3.4mgO2ℓ-1+10.1μgH2Sℓ-1, ≦2.6mgO2ℓ-1 +10.1μgH2Sℓ-1 levels to zoea, megalopa and crab stage larvae, respectively. The results of the field survey in Hiroshima Bay, Japan revealed that a part of the Bay was hypoxia (<3.2 mgℓ-1) in summer with high levels of hydrogen sulfide (≧10μgH2Sℓ-1)near the bottom. From these results, it could be concluded that the low levels of dissolved oxygen and high hydogen sulfide in the bottom water would affect the natural mortality of crab larvae in Hiroshima Bay.