A method is presented for the numerical calculation of a scalar wave propagating along a two-dimensional rough-sided or irregular waveguide. In this situation the wave becomes multiply scattered, with simultaneous interaction at the two boundaries. The field can be expressed in terms of a pair of coupled integral equations; these are derived and solved in an approach based on the parabolic integral equation method, which assumes that all energy is carried in a forward direction. An extended formulation encompassing backscatter is also derived, and a method given for its treatment. This paper serves in part to explain the computational results presented in B. J. Uscinski, “High-frequency propagation in shallow water,” J. Acoust. Soc. Am.98, 2702–2707 (1995).