The propagation of nonlinear fronts in a channel flow of two contiguous homogeneous fluids of different densities is considered. Each fluid layer is of finite depth. The study is restricted to steady flows in a frame of reference moving with the front. The full governing equations are integrated numerically. The numerical method is based on boundary integral equation techniques. Although the propagation of waves in two-layer fluids is a classical problem, this is the first time that fronts have been directly computed. The limiting configuration of fronts as their amplitude increases is discussed and shown to depend on whether the front is of elevation or of depression.