Computer simulations of occupant dynamics are ideal for conducting parametric studies evaluating injury countermeasures. A rollover accident was selected from the National Accident Sampling System (NASS) for simulation to validate the Articulated Total Body (ATB) model's capability to predict occupant dynamics during rollover accidents and to gain insight into injury mechanisms. Simulations of both the driver and passenger occupants which may be used in future countermeasure studies are performed. In the selected accident, a pickup truck rolled multiple times, the belted driver had minor injuries and the unrestrained passenger was ejected with fatal injuries. The body properties for both occupants were obtained using the Generator of Body Data (GEBOD) program based on their weights, heights, and sexes. The interior configuration of the vehicle compartment was modeled based on measurements taken from another vehicle of the same model. The vehicle motion was defined using the results from a vehicle simulation, reported separately. The simulation successfully predicted the passenger's ejection and the driver's minimal impacts. The dynamic parameters and contact forces from the simulation are used in a qualitative comparison with the NASS report. While the computer simulations may not exactly represent the detailed events in the actual accident, they do provide likely occupant responses that can be used in parametric studies investigating injury countermeasures.