Rift Valley fever (RVF), which was enzootic in Sudan in 1976 and epidemic in Egypt in 1977-78, may have been introduced into Egypt from Sudan via sheep transported along Lake Nasser. A hypothesis is presented which describes sheep transport from holding areas in north-central Sudan, where RVF was epizootic, to live animal markets in southern Egypt. Travel time from north-central Sudan to the Aswan area was less than 5 days, approximating the incubation period of RVF virus in sheep. Reintroduction of RVF or other diseases from Sudan into Egypt will be facilitated by the construction of new roads linking the two countries.