The three-dimensional structure of scallop sarcoplasmic reticulum membranes has been determined from electron micrographs of two classes of stain-filled tubules by helical reconstruction methods. These structures are characterized by dimer ribbons of Ca2+-ATPase molecules running diagonally around the tube wall. Deep right-handed grooves separate the ribbons. The elongated, curved units of the dimer (approximately 95 A long in the radial direction; 60 to 70 A axially, and about 30 A wide) are displaced axially by approximately 34 A and are connected at their outer ends by a bridge running nearly parallel to the tube axis. The monomers make a second contact at their inner ends. Adjacent units with the same orientation form a strong contact that is responsible for the ribbon appearance. Comparison of tubules of different diameter shows that one set of connections between the dimer ribbons is conserved: the inner ends of axially displaced dimers appear to make contact along a left-handed path almost perpendicular to the major grooves. The lipid bilayer cannot be clearly identified. The two-dimensional map obtained from flattened tubules is consistent with the three-dimensional reconstruction in showing dimer ribbons connected by a weak contact across the grooves, strongly resembling the inter-dimer bond observed in three dimensions. The two-dimensional map shows a 2-fold axis relating units of the dimer, but the three-dimensional tubes show a slight axial polarity that may arise from the presence of proteins other than the Ca2+-ATPase.