Extensional and shear viscosity of acidified amaranth starch-sodium caseinate suspensions were evaluated. Mixed systems of amaranth starchsodium caseinate acidified with glucone-delta-lactone (GDL) were studied using rheological measurements under biaxial compression and shear. The effects of the acidification rate (slow and fast) and final pH (neutral and isoelectric point of casein) were evaluated considering the interactions between biopolymers and their influence on the rheological parameters. All samples showed shear thinning behavior, but the addition of sodium caseinate in the starch suspensions at neutral pH promoted a negative effect on the apparent viscosity. The acidified samples showed an increase in the complexity of the system due to the formation of a network of starch-casein, but the force required to flow was always higher for samples containing higher concentrations of caseinate. 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