In the 1890s, the Pasteur Institute established a network of laboratories that stretched across France’s empire, from Indochina to West Africa. Quickly, researchers at these laboratories became central to France’s colonial project, helping officials monopolize industries, develop public health codes, establish disease containment measures, and arbitrate political conflicts around questions of labor rights, public works, and free association. Pasteur’s Empire shows how the scientific prestige of the Pasteur Institute came to depend on its colonial laboratories and how, conversely, the institutes themselves became central to colonial politics. This book argues that decisions as small as the isolation of a particular yeast or the choice of a laboratory animal could have tremendous consequences on the lives of Vietnamese and African subjects, who became the consumers of new vaccines or industrially fermented intoxicants. Simultaneously, global forces, such as the rise of international standards and American competitors, pushed Pastorians to their imperial laboratories, where they could conduct studies that researchers in France considered too difficult or controversial. Chapters follow not just Alexandre Yersin’s studies of the plague, Charles Nicolle’s public health work in Tunisia, and Constant Mathis’s work on yellow fever in Dakar, but also the activities of Vietnamese doctors, African students and politicians, Syrian traders, and Chinese warlords. It argues that a specifically Pastorian understanding of microbiology shaped French colonial politics across the world, allowing French officials to promise hygienic modernity while actually committing to minimal development.