The development and implementation of a Fortran computer program called OGSDP to design a least-cost gravity sanitary sewer system are presented. OGSDP is a model that finds the least-cost design for a non-looping, gravity sanitary sewerage system for a given set of design parameters, costs and lay-out. The final design is the least-cost combination of pipe sizes, upstream and downstream invert elevations (of each pipe) and the corresponding manhole sizes. OGSDP respects most Ten States Standards' (1978) design guidelines. OGSDP determines an initial sewer system design using a heuristic procedure, and then improves the design using discrete dynamic programming with successive approximations to obtain the final least-cost design. It is a practical and efficient tool in the design of sanitary sewer systems.