Current industrial trends favor distributed manufacturing with extensive use of subcontracting. In this context we present a two—level decision support system for operations scheduling where a set of firms (subcontractors) is competing for the realization of the manufacturing operations pertaining to a product. We consider the case where operation precedences are given in form of an assembly tree. Each firm bidding for an operation specifies a start time, a processing time, a production cost together with a storage cost for storing the work-in-process resulting from that operation beyond its completion time. At the first level, IBIS, an information support system is used for bid preparation based on resource usage, production requirements and production operations costs. At the second level, given the bids we present a procedure for finding the assignment of operations to firms that minimizes production and storage costs.