We investigate numerically the interactions of two-dimensional jets of Bingham plastic and Herschel-Bulkley fluids with a vertical surface at a distance from the die exit. This problem simulates the early stages of filling of a planar cavity. Our main objective is to explain the flow instabilities observed during the processing of semisolid materials. The effects of the Reynolds and Bingham numbers and of the inlet boundary conditions on both the filling and the stability of the jet are established by means of numerical time-dependent calculations. (C) 2003 Elsevier B.V. All rights reserved.