Two monoclonal antibody neutralization resistant (MAbR) variants of the yellow fever (YF) 17D-204 vaccine virus strain were selected using YF type-specific MAb B39. These B39R variants were compared with the variant virus selected by Lobigs et al. (Virology 161, 474-478, 1987) using a second YF-type specific MAb (2E10) which mapped to amino acid position 71/72 in the envelope (E) protein. Neutralization assays with a panel of MAbs suggested that these two YF-type-specific epitopes are located in two discrete regions of the folded E protein. Each of the B39R variants had a single nucleotide mutation which encoded an amino acid substitution at either position E-155 or E-158. Thus, YF type-specific epitopes map to both domain I (B39) and II (2E10) of the YF virus E protein. The B39 defined epitope represents the first flavivirus neutralizing epitope localized to this region of domain I of the E protein.