The Lwo language, a cluster of dialects belonging to the Western branch of the Nilotic family, was spoken as a mother tongue at the end of the colonial period by some two and a half million people scattered over a vast area of the Upper Nile basin from the Shilluk country in the Sudan to central Uganda and from northeast Zaire to western Kenya. The several components of this stock vary greatly in size, from the five thousand people who comprise the Bor groups of the western Bahr el Ghazal to the Kenya Luo group with well over a million members. Despite their extraordinary numbers, however, the Luo--like the Bor, the Shilluk, the Anywak, and other groups in the Sudan--form an enclave among peoples of quite different speech; the main continuous block of Lwo-speakers is in northern Uganda, consisting of the Alur, Acholi, and Langi peoples, together with the Jo-pa-Lwo (or Chope) group, who live in the corner of the Bunyoro district that is formed by the Victoria Nile as it flows north and then west from Lake Kyoga to the Albert confluence.
This peculiar configuration, very different from the normal pattern of linguistic fragmentation in sub-Saharan Africa, offers historians a problem and an opportunity, for it must be the residuum of an unusual event or series of events. Languages cannot travel unless people carry them; and so it is reasonable to assume that the Lwo-speakers of a few centuries ago lived as a compact community, which developed an exceptional capacity both for the incorporation of aliens and the propagation of its own genes.