We first discuss respective advantages of language interaction in virtual worlds and of using 3D images in dialogue systems. Then, we describe an example of a verbal interaction system in virtual reality: Ulysse. Ulysse is a conversational agent that helps a user navigate in virtual worlds. It has been designed to be embedded in the representation of a participant of a virtual conference and it responds positively to motion orders. Ulysse navigates the user's viewpoint on his/her behalf in the virtual world. On tests we carried out, we discovered that users, novices as well as experienced ones have difficulties moving in a 3D environment. Agents such as Ulysse enable a user to carry out navigation motions that would have been impossible with classical interaction devices. From the whole Ulysse system, we have stripped off a skeleton architecture that we have ported to VRML, Java, and Prolog. We hope this skeleton helps the design of language applications in virtual worlds.