The GP(1,2) envelope glycoproteins (GP) of filoviruses (marburg- and ebolaviruses) mediate cell-surface attachment, membrane fusion, and entry into permissive cells. Here we show that a 151-amino acid fragment of the Lake Victoria marburgvirus GP1 subunit bound filovirus-permissive cell lines more efficiently than full-length GP1. An homologous 148-amino acid fragment of the Zaire ebolavirus GP1 subunit similarly bound the same cell lines more efficiently than a series of longer GP1 truncation variants. Neither the marburgvirus GP1 fragment nor that of ebolavirus bound a nonpermissive lymphocyte cell line. Both fragments specifically inhibited replication of infectious Zaire ebolavirus, as well as entry of retroviruses pseudotyped with either Lake Victoria marburgvirus or Zaire ebolavirus GP(1,2). These studies identify the receptor-binding domains of both viruses, indicate that these viruses utilize a common receptor, and suggest that a single small molecule or vaccine can be developed to inhibit infection of all filoviruses.