Abstract Molecules of bovine fibrinogen and small oligomers of thrombin-induced fibrin have been examined by electron microscopy after contrast enhancement by rotary shadowing and by negative staining. Rotary-shadowed molecules, in addition to exhibiting the well-known trinodular structure, showed a spine of fairly uniform width connecting the central nodule with the terminal protrusions. In specimens negatively stained with uranyl formate or acetate most of the molecules had a gently sigmoidal configuration, all of like handedness. The spines were visibly prominent and frequently had a slight central bulge. The terminal protrusions appeared with a distinctly oblong shape and were oriented parallel to each other, with the long dimension tilted about 45 ° to the axis of the contiguous part of the spine. After exposure of the fibrinogen molecules to millimolar Ca 2+ the central bulge became more noticeable and each terminal protrusion took on the form of two contiguous, equal-sized spheres. Significant dimensions are: overall length of molecule, 45 nm; length and width of spine, 31 nm × 3 nm; length and width of terminal protrusions, 9 nm × 4 nm; diameter of Ca 2+-induced spheres, ~ 4 nm. Specimens from fibrinogen solutions to which minute amounts of thrombin had been added (and the macroscopic clots removed) contained oligomeric assemblies that were small enough to be resolvable into their fibrin subunits. They consisted of two chains that were parallel and were similar to each other in that they never differed by more than one in their subunit number. The elements in one chain of an oligomer appeared to be bound to those of the other chain with a half-molecule staggered overlap. They retained the lengths they had as fibrinogen molecules.