<p>The Whole Effluent Toxicity (WET) test using freshwater organisms has already been validated for freshwater effluent management. Since the effluent is discharged directly into the coastal areas in Japan, there is an urgent need to establish a WET protocol for marine organisms. The availability of test organisms is important for the WET test, but unlike freshwater organisms, breeding and propagation of marine organisms is often difficult. Although the marine crustacean Tigriopus japonicus is easy to keep and breed, it has been considered to have low sensitivity to chemicals. In this study, we developed a test method to improve the sensitivity of this species. The effect of chemical substances was evaluated by the rate of metamorphosis of nauplii (within 24 h after hatching) into copepodite larvae. Additionally, we compared the EC₅₀ values in this study with those from the U.S. Environmental Protection Agency’s (USEPA) database (ecotoxicity data). The mean EC₅₀ was 2.2 (Range: 0.8–4.7) mg/L for hexavalent chromium (measured concentration) and 0.5 (Range: 0.3–0.8) mg/L for 3,5-dichlorophenol (measured concentration). These values were smaller than 93% in hexavalent chromium and 78% in 3,5-dichlorophenol of the ecotoxicity data for marine crustaceans, indicating that the sensitivity of T. japonicus was high. To clarify the usefulness of this test method, it will be necessary to apply the developed test method to other chemicals and effluent.</p>