This paper presents a federated directory system, which allows registration and discovery of semantic web services. The typical use of this directory system is in a context where ubiquitous business application services should be flexibly coordinated and pervasively provided to the mobile user by intelligent agents in dynamically changing environments. The system has been modeled, designed and implemented as a backbone directory system to be searched by an infrastructure made up by such kind of agents coordinating web services. The system is modeled as a federation: directory services form its atomic units, and the federation emerges from the registration of directory services in other directory services. Directories are virtual clusters of service entries stored in one or more directory services. To create the topology, policies are defined on all possible operations to be called on directories. For instance, they allow for routed registration and selective access to directories. WSDir has been applied as a backbone in the trials of an ehealth emergency application.