Force measurements provide an excellent tool to study the folding and unfolding of single-filamentous proteins. In particular, elastic properties of muscle proteins and proteins of the extracellular matrix are of paramount interest for the understanding of structure-function relationships of fibrous proteins. In this study, we focused on the unfolding of native fibronectin, a modular extracellular matrix protein comprising different types of repeating units exhibiting a large variety of functions. Native fibronectin (220-250 kDa) from blood plasma is a dimer composed of at least three classes of repeating units, FN-I, -II, and -III domains. The domains differing in the number of amino acids all have a beta -barrel structure. Statistical analysis of force-extension curves clearly revealed the distinct unfolding of FN-I (45 amino acids), FN-II(60 amino acids), and FN-III(SO amino acids). Besides data about mechanical properties of the filament, the analysis provided also information about the absolute composition and the actual number of amino acids of each domain. Moreover, information about the potential topology may be inferred from computer simulations.