The MIX multiagent architecture has been conceived as a general purpose distributed framework for the cooperation of multiple heterogeneous agents. This architecture, starting from previous work in our group on multiagent systems, has been redesigned and implemented within a research project investigating a particular class of hybrid systems: those integrated by connectionist and symbolic components. This paper describes in some detail the principal concepts of the architecture: the network model and the agent model. Around these models, a set of languages and tools have been developed. In particular, an Agent Description Language (MIX-ADL) has been designed to specify agents declaratively in a hierarchy of classes.