The structure of co-polymers of fibrinogen (F) with N-terminal disulfide knot of desAABB fibrin (tN-DSK) was investigated by transmission electron microscopy of their negatively contrasted samples. It was shown that the co-polymers initial forms were represented by short threads of 2–4 end-to-end arranged F molecules, in which tN-DSK molecules could't be visualized by the used method. Then long single-threaded structures formed which aggregated laterally in fibril-like bunches of varying thickness. The network of fibrils have arisen finally. In control experiments when in the fibrinogen containing samples tN-DSK was substituted for thrombin, one could observe the double-threaded twisted protofibrils but not any of the single-threaded end-to-end arranged molecules. These studies confirm the hypothetical model of F-tN-DSK co-polymer structure as the thread of end-to-end arranged fibrinogen molecules, the adjacent D-domain of which are bound by tN-DSK molecules via the fibrin polymerization sites. The lateral aggregation of co-polymer strains into fibrils and fibril network formation revealed in this study is the evidence of similarity between F-tN-DSK co-polymers and. fibrin polymer structures. It is not wonder because both structures are formed by the unique self-assembly mechanism in which the noncovalent binding of fibrin(ogen) molecules by the complementary fibrin polymerization sites is involved.