Two experiments were designed to understand the performance characteristics of human interaction with a virtual environment. In the first experiment, the effects of control feel (isometric vs. isotonic 3D input device) on tracking performance were measured in a virtual world. The results showed that the isometric device gives better control performance than the isotonic device. In the second experiment, the hypothesis that increased visual depth cues can improve 3D control performance was tested. The presence of binocular depth cues improved tracking performance. However, tracking performance was not significantly improved under perspective depth cues.