ABSTRACT: A dense community of benthic animals was discovered by the Japanese ROV 'Kaiko' in the hadal zone near the bottom of the Japan Trench, 7326 m deep. The community was dominated by a new species of thyasirid bivalve Maorithyashadalis. This community appears to be sustained by chemosynthesis (nutrients being derived from reduced compounds within the sediment) for reasons including: a high concentration of sulfur contained in the thyasirid gills; the existence of numerousbacteria-like particles in the gill tissues; a sulfide smell from soft body parts and from collected sediment; the anoxic nature and high sulfide content of the sediments suggested by the dark gray to black color; the occurrence above geologic faults.Thyasirids have also been collected from other chemosynthesis-based communities around Japan, including the Japan Trench, Sagami Bay and the Nankai accretionary prism. Until now, the deepest chemosynthesis-based community known occurred on the SanrikuEscarpment of the Japan Trench, 6437 m deep. This site was dominated by the vesicomyid clam Calyptogena phaseoliformis. The discovery of a chemosynthesis-based community dominated by thyasirid clams from even deeper waters suggests a widervariety of chemosynthesis-based communities exists throughout deep-sea trenches. Additionally, the thyasirid species discussed here differs from other thyasirids by having circular symbionts which appear to exist intracellularly, and by living exposedabove the sediment surface.