In this paper we discuss a distributed user model architecture suitable for hypermedia systems. In this architecture, the user knowledge is spread over a collection of autonomous actors, cooperating in a way which provides, through collaborative activities, a global, dynamic user model. Separate duties and knowledge, combined with asynchronous and concurrent processing, augment the adaptively and the efficiency of the platform, avoiding the drawbacks of more traditional centralized approaches. Our goal was to enrich an existing hypermedia architecture with an appropriate user model activity, conceived and realized with the same design perspective of the underlying system, the actor-level design approach. The organizational structure has been defined by extending a previous hypermedia system model without redesigning the overall architecture. The prototype has been tested with success as an adaptive hypermedia interface in a logic object- oriented programming environment.