This essay reflects upon the milieu and the character of Brazilian and Argentinean natural history museums during the second half of the nineteenth century. It argues that the museums were influenced not only by European and North American museums but by each other. Museum directors in the two countries knew each other and interacted. Some of the relationships between these museums were friendly and cooperative, but because they were in young, emerging nations, they also became deeply involved in the invention of nationality in their respective countries and interacted as rivals and competitors. Even through rivalry, however, they contributed to each other's development, as did rivalry among museums within each of the two countries. Later in the century they went well beyond the nationalist perspective, finding, through their research into paleontology and anthropology in their regions, a continental and uniquely South American scientific perspective, defined in reaction to North American and European views.