We identified the two-stranded fibrin protofibril and studied its structure in electron micrographs of negatively stained specimens. Based on these images and on considerations of symmetry, we constructed a model of the protofibril in which the two strands of trinodular fibrin molecules are related by a two-fold screw axis between the strands and two-fold axes perpendicular to them. The two strands are held together by staggered lateral contacts between the central nodules of one strand and outer nodules of the other. The molecules within a strand are joined by longitudinal contacts between outer nodules. This interpretation of the structure of protofibrils is supported by images of trimer complexes whose preparation and structure are described here, in which the central nodule of a fibrin monomer is attached to the crosslinked outer nodules of two other molecules. We conclude that the association of protofibrils to form thicker fibers must involve a second type of lateral contact, probably between outer nodules of adjacent, in-register strands. In total, we identify three intermolecular contacts involved in the polymerization of fibrin.