Synthesis of the structural, surface glycoprotein (GP) of Ebola virus (EBOV) is dependent on transcriptional RNA editing phenomenon. Editing results in the insertion of an extra adenosine by viral polymerase at the editing site (7 consecutive template uridines) during transcription of GP gene of the wild-type virus (EBOV/7U). In this study, we demonstrate that passage of EBOV/7U in Vero E6 cells results in the appearance and rapid accumulation of a variant (EBOV/8U) containing an additional uridine at the editing site in the viral genome. EBOV/8U outgrows and eventually replaces the wild-type EBOV during 4-5 passages. On the contrary, infection of guinea pigs with EBOV/8U leads to the appearance and rapid predominance by EBOV/7U. These rapid conversions suggest that editing of the genomic RNA occurs at a higher frequency than previously thought. In addition, it indicates that the EBOV/7U phenotype has a selective advantage that is linked to controlled expression of GP and/or expression of secreted sGP, the primary gene product for wild-type EBOV. This study demonstrates the potential for insertion and deletion of uridines in the editing site of the EBOV genomic RNA, depending on environmental constraints.