The theory developed by Doi and Ohta was evaluated for its ability to predict the rheology of an immiscible polymer blend. The theory describes the additional stresses arising as a consequence of interfacial tension in two phase systems in which the constituents consist of Newtonian fluids and have equal viscosities. The blend considered in this paper consisted of an immiscible mixture of poly(ethylene terephthalate) (PET) and nylon 6,6 at a composition ratio of 25/75 w/w PET/nylon 6,6. The rheological properties of this blend were found to be stable for the time frame required for the rheological experiments used in this work (e.g., < 5 min). The Doi-Ohta theory was found to be capable of qualitatively predicting the extra stresses arising as a result of the interfacial tension as observed in the steady state viscosity and steady state first normal stress difference. The transient shear stress and first normal stress difference at the start up of steady shear flow were qualitatively predicted by the Doi-Ohta theory while the recovery of the initial overshoot observed experimentally was not. The overshoot observed experimentally during step-up experiments and the undershoot observed during step-down experiments were not predicted by the theory in which it was predicted that the stresses change monotonically with a stepwise change of the shear rate to the final steady state value. While the shear thinning behavior observed for this blend was not predicted by the theory, the scaling relation for the transient stresses predicted by the theory was found to hold for the blend using stepwise changes of shear rate at a constant step ratio. (C) 1996 Society of Rheology.