This paper describes an interactive dynamic simulator for virtual environments which allows user interaction via a haptic interface. The interactive simulation is performed in our testbed dynamic simulator I-GMS, which has been developed in an object-oriented framework for simulating motions of free bodies and complex linkages such as those needed for robotic systems or human body simulation. User-interaction is achieved by performing push and pull operations via the PHANToM haptic device which runs as an integrated part of I-GMS. We demonstrate the user-interaction capability of I-GMS through on-line editing of trajectories for a 6-dof robot manipulator.