To study the environmental impact of the assessment technologies for the development of shallow methane hydrate zones in the Sea of Japan, deep-sea amphipods (Pseudorchomene sp. and Anonyx sp.) were collected from a depth of approximately 1000 m and were tested for H2S toxicity. At 0.57 mg L−1 H2S, all specimens of Pseudorchomene sp. were dead after 96 h, whereas all individuals survived at 0.18 mg L−1. Moreover, Anonyx sp. had a survival rate of 17 % after 96 h at 0.24 mg L−1. A similar toxicity test was conducted with the coastal amphipod Merita sp., a detritivore, and all individuals died within 24 h at 0.15 mg L−1. These results suggested that compared with coastal detritivorous amphipods, deep-sea detritivorous amphipods, which also live near biomats with sediment H2S concentrations exceeding 10 mg L−1, showed a higher tolerance to H2S.