In this paper, I examine the intertwined science, politics, and religion of a major figure of nineteenth-century French biology, the Parisian professor of histology Charles Robin (1821–1885). Historiography generally associates his name with France’s rejection of the cell theory formulated by Schwann and then Virchow in the 1830s–1850s. One of the main factors put forward is the influence of Comtean positivism. Here, I propose to go beyond this historiography and discuss not only convergences but also divergences between Robin’s and Comte’s visions of the organism and society. Moreover, I analyze Robin’s research agenda in light of the political ideas he defended as a republican in the context of the emergence of the Third Republic. At first sight, Robin’s political activity (marked by his late tenure as senator) may initially appear disconnected from his scientific agenda. However, I argue that Robin’s approaches to different areas of knowledge (biology, sociology, politics, and metaphysics) were mutually supportive and lent one another authority, especially through the parallel structure and shared vocabulary of their discourses. Ultimately, I demonstrate that Robin’s biological materialism, combined with his outspoken anticlericalism, constitutes a political stance, and show how the concept of “solidarity” helped him to cast a new light on the relations between the parts and the whole, both in biology and social policy. This essay is part of a special issue entitled REVOLUTIONARY POLITICS AND BIOLOGICAL ORGANIZATION IN NINETEENTH-CENTURY FRANCE AND GERMANY edited by Lynn K. Nyhart and Florence Vienne.