To characterize the structure of jaw muscle fibres expressing masticatory (superfast) myosin, X-ray diffraction patterns of glycerinated fibres of dog masseter were compared with those of dog tibialis anterior in the relaxed state. Meridional reflections of masseter fibres were laterally broad, indicating that myosin filaments are staggered along the filament axis. Compared with tibialis anterior fibres, the peak of the first myosin layer line of masseter fibres was lower in intensity and shifted towards the meridian, while lattice spacings were larger at a similar sarcomere length. These suggest that the myosin heads of masticatory fibres are mobile, and tend to protrude from the filament shaft towards actin filaments. Lowering temperature or treating with N-phenylmaleimide shifted the peak of the first myosin layer line of tibialis anterior fibres towards the meridian and the resulting profile resembled that of masseter fibres. This suggests that the protruding mobile heads in the non-treated masticatory fibres are in the ATP-bound state. The increased population of weakly binding cross-bridges may contribute towards the high specific force of masticatory fibres during contraction. Electron micrographs confirmed the staggered alignment of thick filaments along the filament axis within sarcomeres of masticatory fibres, a feature that may confer efficient force development over a wide range of the sarcomere lengths.