A multimodal presentation planning mechanism must take into consideration the structure of the discourse and the constraints imposed by discourse relations. This requires that different processes that perform multimodal presentation planning be able to communicate with each other. In this article, we introduce a multiagent architecture based on the blackboard system that satisfies this requirement. In addition, we describe a constraint propagation mechanism that transfers plan constraints from one level of the presentation planning process to the next, and we discuss the cooperation and negotiation processes between modality-specific agents in a prototype system that implements the multiagent planning mechanism.