We investigated the influence of salinity (5 ppt versus 25 ppt) on acute (96-h LC₅₀) and chronic toxicity (15–30 day LC₅₀) of Ni in two euryhaline crustaceans, the shrimp (Litopenaeus vannamei) and the isopod (Excirolana armata). 96-h LC50 values were 41 μmol L⁻¹ and 362 μmol L⁻¹ for L. vannamei and 278 μmol L⁻¹ and > 1000 μmol L⁻¹ for E. armata at 5 ppt and 25 ppt, respectively. Speciation analysis demonstrated that complexation with anions such as SO₄²⁻, HCO₃⁻ and Cl⁻ at 25 ppt had a negligible effect on reducing the free Ni²⁺ ion component in comparison to 5 ppt. The salinity-dependent differences in acute Ni toxicity could not be explained by differences in Ni bioaccumulation. Therefore, differences in physiology of the organisms at the two salinities may be the most likely factor contributing to differences in acute Ni toxicity. Chronic LC₅₀ values (2.7–23.2 μmol L⁻¹) were similar in the two species, but salinity had no significant effect, indicating that water chemistry and osmoregulatory strategy do not influence chronic toxicity. However chronic (15-day) mortality in both species could be predicted by acute (96-h) Ni bioaccumulation patterns.