Abstract: This paper describes the integration of the Berkeley Architectural Walkthrough Program with the National Institute of Standards and Technology's CFAST fire simulator. The integrated system creates a simulation based design environment for building fire safety systems; it also allows fire safety engineers to evaluate the performance of building designs, and helps make performance-based fire codes possible. We demonstrate that the visibility preprocessing and spatial decomposition used in the Walkthru also allow optimization of the data transfer between the simulator and visualizer. This optimization improves the ability to use available communication band-width to get needed simulation data to the Walkthru in the best order to visualize results in real time; an appropriate communication model and data structures are presented. General issues arising in the integration of environmental simulations and virtual worlds are discussed, as well as the specifics of the Walkthru-CFAST system, including relevant aspects of the user interface and of the visualization and simulation programming interfaces. A recommendation is made to structure future simulators in such a way that they can selectively direct their computational efforts toward specified spacetime regions of interest and thereby support real-time, interactive virtual environment visualization more effectively.