The viscoelastic behavior of acrylonitrile-butadiene-styrene (ABS) terpolymers is studied in the molten state. First, the behavior of the styrene-acrylonitrile (SAN) matrix is determined. Then, the effect of the degree of grafting on ABS theological properties is emphasized. A critical degree of grafting (DGc), corresponding to a minimum of the viscoelastic functions in the low frequency region, is determined. It defines a critical thickness of the grafted layer, and is related to the conformation of the grafted chains at the surface of the rubber particle. The DGc affects the morphology of the ABS and also affects the appearance of a secondary plateau at low frequencies. The effects of the rubber particles, of the grafted chains, and of the morphology on the appearance of the secondary plateau at low frequencies are clearly dissociated.