This is an account of a zoo-geographic reconnaissance through the Brazilian hinterland. The official and proper title of the expedition is that given it by the Brazilian Government: Expedicao Scientifica Roosevelt- Rondon. When I started from the United States, it was to make an expedition, primarily concerned with mammalogy and ornithology, for the American Museum of Natural History of New York. This was undertaken under the auspices of Messrs. Osborn and Chapman, acting on behalf of the Museum. In the body of this work I describe how the scope of the expedition was enlarged, and how it was given a geographic as well as a zoological character, in consequence of the kind proposal of the Brazilian Secretary of State for Foreign Affairs, General Lauro Muller. In its altered and enlarged form the expedition was rendered possible only by the generous assistance of the Brazilian Government. Throughout the body of the work will be found reference after reference to my colleagues and companions of the expedition, whose services to science I have endeavored to set forth, and for whom I shall always feel the most cordial friendship and regard.