We are presenting the Virtualis project, which is a virtual interactive opera : it is a CD-ROM opera specially designed and composed to be " performed " on any personal computer and proposing possibilities of interaction. Opera can then be accessed at home and no longer in theatres. It is both an artistic project (as an opera creation) and a scientific one (in the field of human-machine interaction). We have first been modelling the interaction situation in traditional opera using an interagent collaboration formalism called MADEINCOOP. We then applied this model to the interactive opera performance, where a user faces a computer. For the interaction model, we have chosen to use an analogy of a system of physical forces (e.g. electrical or gravitation forces). Forces and fields rule the evolution of the positions of the characters on the screen, whereas the user/listener triggers perturbations by his/her actions. We are currently implementing the Virtualis project, using Director and also a specific environment named ALMA we have developped. It enables the creation of fine correspondances between musical fragments (described using GUIDO language) and graphical 3D elements. The user may wander inside music and interact with it, either with sounds or texts.