The internal response over a sill to strong tidal flow superimposed on a vertically sheared steady flow is examined with a simple two-layer analytical model. As in the case where the vertical shear is absent, the upstream propagating internal wave is gradually formed through interference of infinitesimal-amplitude internal waves (elementary waves) emanating from the sill at each instant of time (Hibiya, 1986); with the composite internal Froude number exceeding (falling below) unity in the accelerating (decelerating) stage of the two-layer flow, the upstream propagating elementary waves come close together over the sill. In this case, however, the amplitude of the elementary wave is increased when the lower layer velocity is larger than the upper layer velocity, whereas it is reduced when the upper layer velocity is larger than the lower layer velocity. As a result, the internal wave field over the sill differs markedly between the ebb and the flood tidal phases. It follows from this analytical result that in the Strait of Gibraltar, the internal wave propagating Mediterraneanward is significantly amplified during the ebb (westward) phase of the tidal flow, whereas such amplification does not take place throughout the tidal cycle for the internal wave propagating Atlanticward. This is consistent with the result from satellite observation that the sea surface signatures caused by underlying large-amplitude interval wave trains appear only on the Mediterranean side and not on the Atlantic side. ¿ American Geophysical Union 1990