Existing studies on heuristic lot-sizing techniques for multistage production systems concentrate on optimizing the production operations of the individual stages. This approach induces a family of heuristics, in which the production plans for the individual stages are computed sequentially starting from the final stage. In this paper we introduce a parallel heuristic for the problem. The method, which may be considered as the generalization of the single stage Wagner-Whitin algorithm, attempts to optimize over all stages simultaneously, while building the production plans in a forward manner. In computational experiments the new algorithm performed extremely well.