Reports of nonhomologous recombination-events between persistently infecting and vaccinal strains of Bovine virus diarrhea virus (BVDV) indicate that mixed pestivirus infections occur in vivo and that bovine cells support replication of RNA from BVDV-I and BVDV-2 (Ridpath and Bolin. 1995: Becher et al. 2001). BVDV type-specific molecular beacons were used to estimate the effect of the presence of two BVDV RNA genotypes on the RNA relative replication efficiency of each genotype by comparing Ct values obtained from the samne reaction tube in a real-time RT-PCR. This was done in bovine and ovine cultured cells following simultaneous- or superinfection with different biotypes and genotypes of BVDV. The noncytopathic BVDV-2 890 blocked superinfection with the cytopathic BVDV-I strain Singer in both BT and MDOK cells. The cytopathic BVDV strains greatly reduced the relative concentration of superinfecting but not coinfecting cytopathic and noncytopathic BVDV-2 RNA in BT cells. On the Other hand. superinfection of Singer infected cells with BVDV-2 890 increased BVDV-2 890 RNA relative concentration in MDOK cells.