Toughness and collagen content were measured for various muscle parts of the Japanese abalone, kuro-awabi (Haliotis discus), in relation to muscle structures. The dorsal surface of the foot was toughest, followed by the hard and soft part of the foot, then the upper and middle part of the adductor muscle, irrespective of being reared or wild specimens. When compared with other abalone species, kuro-awabi showed the highest toughness for all the muscle parts, followed by madaka (H. sieboldii) and megai-awabi (H. gigas), while ezo-awabi (H. discus hannai) was softest. Collagen content was parallel with muscle toughness: the higher the collagen content, the tougher the muscle. Light micrographs of kuro-awabi showed that foot and the dorsal surface of foot were dominated by connective tissues, while adductor muscle was mainly composed of myofibrils. Transmission electron micrographs demonstrated that myofibrils in the foot were surrounded by thick layers of collagen fibrils of about 1 μm, confirming light microscopic observations.