The Swiss physician and naturalist Conrad Gessner (1516–1565) is known as the author of Historia animalium, a multi-volume encyclopaedia published between 1551 and 1558 and intended as an up-to-date version of the Aristotelian work of the same title. It included little-known animals from the New World and other regions outside Europe. To realize this ambitious project, Gessner was dependent on a great number of supporters and informants. One of them was the English physician John Caius (1510–1573), who shared with Gessner a special interest in the medical works of Galen. This common interest resulted in a meeting between the two scholars, leading to cooperation and a life-long friendship. The fact that Caius and Gessner were on good terms and cooperated for Historia animalium, as well as for Gessner's unfinished “Historia plantarum”, has often been noted, usually however in a rather cursory manner. This article provides an analysis of how and when Caius's information found its way into Gessner's works.