ABSTRACT Negative-strand RNA viruses encode a single RNA-dependent RNA polymerase (RdRp) which transcribes and replicates the genome. The open reading frame encoding the RdRp from a virulent wild-type strain of rinderpest virus (RPV) was inserted into an expression plasmid. Sequences encoding enhanced green fluorescent protein (EGFP) were inserted into a variable hinge of the RdRp. The resulting polymerase was autofluorescent, and its activity in the replication/transcription of a synthetic minigenome was reduced. We investigated the potential of using this approach to rationally attenuate a virus by inserting the DNA sequences encoding the modified RdRp into a full-length anti-genome plasmid from which a virulent virus (rRPV ^KO ) can be rescued. A recombinant virus, rRPV ^KO L-RR egfp R, which grew at an indistinguishable rate and to an identical titer as rRPV ^KO in vitro, was rescued. Fluorescently tagged polymerase was visible in large cytoplasmic inclusions and beneath the cell membrane. Subcutaneous injection of 10 ⁴ TCID ₅₀ of the rRPV ^KO parental recombinant virus into cattle leads to severe disease symptoms (leukopenia/diarrhea and pyrexia) and death by 9 days postinfection. Animals infected with rRPV ^KO L-RR egfp R exhibited transient leukopenia and mild pyrexia, and the only noticeable clinical signs were moderate reddening of one eye and a slight ocular-nasal discharge. Viruses that expressed the modified polymerase were isolated from peripheral blood lymphocytes and eye swabs. This demonstrates that a virulent morbillivirus can be attenuated in a single step solely by modulating RdRp activity and that there is not necessarily a correlation between virus growth in vitro and in vivo.