This paper describes a study of a three-dimensional virtual prototype intended for usability testing and concept validation over the Internet. A virtual reality modeling language (VRML)-model of a fictive mobile phone concept with simulated interaction functionality was tested for usability following two approaches. The first group performed a traditional task-based usability test; the other performed a free exploration test. The approaches were compared e.g. by asking the participants to draw their mental model of the product's user interface. The research aimed at supporting de-centralized product development. The goal was to expand the possibilities of user-centered design methods by utilizing the Internet. The results indicate that 3D virtual reality prototypes can be used for remote usability testing and design evaluation. The limitations and requirements for successful prototyping and testing are discussed. The task-based and free exploration testing approaches are also compared. Moreover, broader issues on usability testing using virtual reality (VR) models compared to physical models are discussed, for example how users experienced the product from the VR model compared to a physical appearance model.