In multiagent planning, an agent sometimes needs to collaborate with others to construct complex plans, or to accomplish large organizational tasks which it cannot do alone. Since each agent in a group may have incorrect beliefs about the world and incomplete knowledge, and because agent's abilities differ, constructing a coordinated collaborative plan among agents is a difficult proposition. In previous work [Osawa and Tokoro 92], we developed a scheme for constructing collaborative plans from the. possibly incomplete, individual plans of agents. This scheme was designed to provide availability-based assignment of goals to agents, and opportunistic collaboration to distributed planning in open multiagent environments based on the contract net. In this paper, we formalize incomplete individual plans and collaborative planning among rational agents using the Multi-World Model, and provide a utility-based model for rational choice of actions. Agents can effectively balance workload...