The antiviral state induced by alpha/beta interferon (IFN-α/β) is a powerful selective pressure for virus evolution of evasive strategies. The paramyxoviruses simian virus 5 (SV5) and human parainfluenza virus 2 (HPIV2) overcome IFN-α/β responses through the actions of their V proteins, which induce proteasomal degradation of cellular IFN-α/β-activated signal transducers and activators of transcription STAT1 and STAT2. SV5 infection induces STAT1 degradation and IFN-α/β inhibition efficiently in human cells but not in mouse cells, effectively restricting SV5 host range. Here, the cellular basis for this species specificity is demonstrated to result from differences between human and murine STAT2. Expression in mouse cells of full-length or truncated human STAT2 cDNA is sufficient to permit antagonism of endogenous murine IFN-α/β signaling by SV5 and HPIV2 V proteins. Furthermore, virus-induced STAT protein degradation is observed in mouse cells only in the presence of ectopically expressed human STAT2. The results indicate that STAT2 acts as an intracellular determinant of paramyxovirus host range restriction, which contributes to the species specificity of virus replication, and that human STAT2 can confer a growth advantage for SV5 in the murine host.