The author argues that the idea which came to be prevalent from the beginning of the Christian Era that the Nile springs from the mountains of Atlas goes back to Polybius. Section 1 introduces two ancient theories (let us call them «meteorological» and «solar») which explained why the equatorial zone is inhabitable. It is argued that the first author to formulate the «solar» theory was Posidonius, who advanced it against the «meteorological» theory suggested by Polybius. In section 2, it is argued that Polybius' theory presupposed that the sources of the Nile were situated near the equator. In section 3, the analysis of three obscure Posidonius' remarks concerning his «solar» theory makes it possible to conclude (1) that Polybius located the Nile's sources in Western Africa, and (2) that it was this idea that induced Posidonius to reject the «meteorological» theory. Section 4 deals with the ancient theory which placed the sources of the Nile at the Atlas. The author argues that this theory must have existed already in the 2nd century B.C. In section 5, the author shows that the untraditional localization of Atlas in Mela, Pliny, and Pausanius, which is closely connected with the theory at issue, goes back to Polybius. This conclusion makes it possible to assume that the very idea of the Mauritanian origin of the Nile went back to Polybius as well, and was closely related to his «meteorological» theory.