Thin-layer theory is developed for Bingham plastic fluids; the specific case of a fluid flowing down an inclined plane is considered. In contrast to previous work it is indicated how the Bingham model leads directly to a self-consistent thin-layer theory; this does not rely upon adopting a bi-viscous approximation. The theory describes the fluid in terms of regions of fully plastic flow bounded by a fake yield surface. Above this fake yield surface are ＇pseudo-plugs＇ - regions in which the leading-order equations predict a plug, but which are seen to be weakly yielded at higher order.