A computational fluid dynamics (CFD) model of fire and smoke transport is described. Combustion is represented by means of a single conserved scalar known as the mixture fraction. Radiation transport is approximated in the gray gas limit. The algorithms have been incorporated in the Fire Dynamics Simulator (FDS), a computer program maintained by the National Institute of Standards and Technology. Sample calculations are presented demonstrating the performance of the new algorithms, especially as compared to earlier versions of the model.