Multimodal interactive systems support multiple interaction techniques such as the synergistic use of speech and direct manipulation. The flexibility they offer results in an increased complexity that current software tools do not address appropriately. One of the emerging technical problems in multimodal interaction is concerned with the fusion of information produced through distinct interaction techniques. In this article, we present a generic fusion engine that can be embedded in a multi-agent architecture modelling technique. We demonstrate the fruitful symbiosis of our fusion mechanism with PAC-Amodeus, our agentbased conceptual model, and illustrate the applicability of the approach with the implementation of an effective interactive system: MATIS, a Multimodal Airline Travel Information System.