This paper is concerned with the rheological modeling of untreated carbon nanotubes (CNTs) suspended within an epoxy resin. The untreated CNT suspensions exhibited significant steady shear thinning and contained optically resolvable aggregate structures. A simple orientation model, based on a Fokker-Planck advection-diffusion description, failed to capture the experimentally observed rheological responses for untreated CNT suspensions. A new model named the "aggregation/orientation" (AO) model has been developed to describe the experimental findings. The model integrated elements of both a standard orientation model and aggregation modeling concepts within the Fokker-Planck formalism. A hierarchy of states between CNTs that are free from entanglement and a complete CNT network was incorporated into the AO model, thereby enabling different microstructure populations to exist for different shear conditions. Using a small number of adjustable parameters, it was found that the experimental data could be fitted with reasonable precision.