Parallelism for gray participating-media radiation heat transfer may be placed in two primary categories: spatial and angular domain-based parallelism. Angular-based decomposition is limited for large-scale applications, however, given the memory required to store the spatial grid on each processor. Therefore, the objective of this work is to examine the application of spatial domain-based parallelism to large-scale, three-dimensional, participating-media radiation transport calculations using a massively parallel supercomputer architecture. Both scaled and fixed problem size efficiencies are presented for an application of the discrete ordinate method to a three-dimensional, nonscattering radiative transport application with nonuniform absorptivity. The data presented show that the spatial domain-based decomposition paradigm results in some degradation in the parallel efficiency but provides useful speed-up for large computational grids.