Abstract Samples of butterflies were collected along the Zaire River from its source to Kisangani and in the Ituri region. The number of species flying in the same area has been estimated wherever possible, and the degree of faunal similarity at different sites. The number of species increases as one proceeds north from 50–100 to 100–150, and the fauna is separable into two groups. The difference in faunal composition and diversity is related to the southern boundary of the rain forest. The comparative poorness of the butterfly fauna in tropical Africa compared to tropical South America is discussed, with reference to the parallel difference in the bird faunas of the two continents.