Medical Entomology emerged in Brazil in the late nineteenth century, through the initiative of a group of physicians dedicated to researching microorganisms related to diseases of public health importance, especially yellow fever and malaria. They led the institutionalization of Bacteriology and Tropical Medicine in southeast Brazil and the sanitation of coastal cities and, subsequently, rural areas. Medical Entomology provided the professionals who would undertake campaigns against agricultural plagues, as well as the institutionalization of Agronomy and Veterinary Medicine. In the present article, I intend to show how relations between the professionals who gave life to Medical Entomology in Brazil were interwoven and to illustrate their relations with entomologists in other countries. I will also present an overview of the research problems faced by Brazilian entomologists at the turn of the nineteenth century and early decades of the twentieth.