Fisher's virulent positions on such themes as "race degeneration", the necessity of sterilization measures for certain categories of the American population, and his urgent call for the control of the genetic quality of new immigrants are hardly consistent with the opalescent subtlety of academic disputes over the nature of capital and interest. Although Fisher repeats it often: in his work, this question of the nature of capital and interest is directly linked to eugenic assumptions and analysis. This second body of Fisherian work illuminates the strong epistemological and theoretical references in Fisher's work as an economist. This paper addresses this question through three major themes: the constant denunciation of a "racial decay" of the American population and its corollary: the project of setting up a "scientific humaniculture"; the plea against the eugenic effects of World War I, and the then haunting question of the closing of the "Golden Door".