The present invention comprises new pestivirus mutants, characterized in that the mutants are based on a cp strain of the virus wherein part of the gene sequence encoding the Npro region is deleted, except that the deleted part does not encompass the coding sequence for the N-terminal twelve amino acids of the Npro protein. Preferably the pestivirus is the Bovine Viral Diarrhea Virus (BVDV). It was found that a mutant according to the invention is a safe and efficient vaccine candidate.