An epizootic of Rift Valley fever (RVF) occurred in Egypt between April and August 1997. The signs among infected cattle and sheep were high fever, icterus, bloody diarrhoea and abortion. Aborted sheep foetuses and sera from the affected herds were collected in the Aswan and Assiut Provinces, Upper Egypt, for virological and serological examination. A cytopathic effect was detected in Vero cell cultures 48 h after inoculation with the foetal liver and spleen suspensions. The same suspensions caused paralysis and mortalities two to three days post intracerebral injection in mice. The isolated virus was identified using an agar gel precipitation test (AGPT) and a direct fluorescent antibody technique. Serological examination revealed that all tested sheep (57) and cattle (93) gave positive results to serological tests, using a complement fixation (CF), serum neutralisation (SN) and indirect immunofluorescence assay; while only 48 (84.2%) out of 57 sheep sera and 69 (74.2%) out of 93 cattle sera gave positive results using an AGPT. Titration of the serum samples indicated that SN is more sensitive than CF. Importation of infected ruminants, especially camels from the Sudan, is the principal source of infection. Aswan, the nearest Egyptian province to the Sudan, is the focus of RVF virus infection in Egypt. As a result of high insect populations, the epizootics of RVF have usually occurred during the summer in Egypt. Reoccurrence of epizootics from time to time indicates failure of the applied RVF vaccination programme in Egypt.