Thick filaments have been isolated from the striated adductor muscle of the scallop and examined by electron microscopy after negative staining. Many filaments appear intact, and reveal a centrally located bare-zone and a well-defined helical surface array of myosin crossbridges characterized by a 145 A axial period and prominent helical tracks of pitch 480 A. Heavy-metal shadowing shows that these helices are right-handed. A small perturbation of alternate crossbridge levels produces an axial period of 290 A, which is most prominent in a region on either side of the bare-zone. Image analysis reveals that the crossbridge array has 7-fold rotational symmetry, one of the possibilities suggested by earlier X-ray diffraction studies of native filaments in scallop muscle. A low-resolution three-dimensional reconstruction shows elongated surface projections ("crossbridges") that probably represent unresolved pairs of myosin heads. They run almost parallel to the filament surface, but are slewed slightly from the axis so that they lie along the right-handed helical tracks of pitch 480 A. The connection to the filament backbone probably occurs at the end of the crossbridges nearer the bare-zone; thus, their sense of tilt appears to be opposite to that of rigor attachment to actin. The 290 A period arises from a different distribution of crossbridge density at alternate levels; in addition, there are weak connections between the top of one crossbridge and the bottom of the next, 145 A away. The prominence of the 290 A period near the bare-zone suggests that anti-parallel molecular interactions are mainly responsible for this perturbation.