Acute lethal toxicity of hexavalent chromium on nauplii of the marine harpacticoid Tigriopus japonicus was tested. Nauplii (<24h after hatching) were exposed to five test concentrations from 9.5 to 100mg/L and the control at 25℃ for 24h in three replicates. From the results, the 24-h median lethal concentration with 95% CI was estimated at 83 (49-140) mg/L.