Bovine viral diarrhea virus (BVDV) is a positive-strand RNA virus and a member of the genus Pestivirus in the family Flaviviridae. To identify and characterize essential factors required for BVDV replication, a library expressing random fragments of the BVDV genome was screened for sequences that act as transdominant inhibitors of viral replication by conferring resistance to cytopathic BVDV-induced cell death. We isolated a BVDV-nonpermissive MDBK cell clone that harbored a 1.2-kb insertion spanning the carboxy terminus of the envelope glycoprotein 1 (E1), the envelope glycoprotein E2, and the amino terminus of p7. Confirming the resistance phenotype conferred by this library clone, naïve MDBK cells expressing this fragment were found to be 100- to 1,000-fold less permissive to both cytopathic and noncytopathic BVDV infection compared to parental MDBK cells, although these cells remained fully permissive to vesicular stomatitis virus. This restriction could be overcome by electroporation of BVDV RNA, indicating a block at one or more steps in viral entry prior to translation of the viral RNA. We determined that the E2 ectodomain was responsible for the inhibition to BVDV entry and that this block occurred downstream from BVDV interaction with the cellular receptor CD46 and virus binding, suggesting interference with a yet-unidentified BVDV entry factor.