Unfortunately, this is still unknown. Collection of animal specimens were unsuccessful in determining a natural reservoir that were taken in the wake of the Kikwit outbreak.
Recently (January 2006) Scientists from the IRD and the CIRMF detected specific Ebola virus antibodies in the serum of three species of tropical fruit bats and revealed the presence of viral genome fragments in the liver and spleen of these bats. This discovery should not be confused with finding the reservoir or vector of Ebola - this simply indicates that it is possible that Ebola infected bats could survive and transmit the virus to primates. This does not indicate that bats are the natural reservoir, just that it is a possibility.
The possible species in tropical Africa are so numerous that a long and lucky search is likely to be required.