At the origin of this research, is the conviction that securities transactions performed by banks must not be perceived as a more or less recent innovation but as a permanent feature of banks’ activity emphasizing the unity of the financial system. We first put in perspective the theoretical works on the financial intermediation and the role of banks in the mechanisms of financing. We then describe the main stages of the American national banking system; since its creation in 1863, statutory evolutions have affected both the system and banking practices on securities transactions. We thus analyze the securities transactions for own account of the US commercial banks since 1863 through the transformations of their balance sheet assets. We conclude on the durability of the securities activity of banks but also on its high flexibility.