This paper presents a federated directory system called WSDir, which allows registration and discovery of seman- tic web services. The typical use of this directory system is in a context where ubiquitous business application ser- vices should be flexibly coordinated and pervasively pro- vided to the mobile user by intelligent agents in dynamically changing environments. The system has been modeled, de- signed 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 direc- tory services in other directory services. Directories are virtual clusters of service entries stored in one or more di- rectory 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 ac- cess to directories. WSDir has been applied as a backbone in the trials of an ehealth emergency application.