In the 1940s the renowned American sociologist Robert K. Merton formulated the ethos of science - a set of norms whose use by scientists ensured, in his view, the optimal development of science. This essay examines Merton's four norms - universal-ism, communism, disinterestedness, and organized skepticism - and the debates over their adequacy for understanding the real life of science at different stages of its social history. Particular attention is devoted to the evolving perception of these norms by the scientific community and to the analysis of their role in “little science,” “big science,” and today's science in transition. Although the Mertonian ethos of science does not constitute a categorical imperative for all practitioners of science, it nonetheless remains in the consciousness of its representatives and functions as a “guardian” by supporting the values of basic science and the continuation of fundamental research. Empirical research demonstrates that systems of values corresponding to the Mertonian ethos of science persist in the contemporary scientific community, in particular, among Russian academic scientists.