Airborne microorganisms are a source of a wide variety of public health hazards; of particular significance are aerosols of biological warfare agents (BWA).  Despite the rapid evolution of detection technologies for BWA, important questions remain unanswered concerning the fate and transport of biothreat agent in HVAC ducts. If an agent is released into a building the particles could be transported throughout the building in the HVAC unit.  This project looked at fluidized spore deposition and resuspension using fluidized spores under normal HVAC airflow conditions.  Three common ventilation duct materials were evaluated: flexible plastic, galvanized steel, and internally insulated fiberglass. Transport efficiency ranged from 9 to 13% in steel and fiberglass ducts; transport efficiency was far less (0.1 to 4%) in plastic duct. Results showed that the deposition of spores was significantly different in the three duct materials evaluated. All experimentally determined deposition velocities were in the range of theoretical predictions for rough surfaces. All were 10 to 100 times greater than the velocities predicted for ducts with smoother surfaces. For plastic duct, greater deposition velocities were likely the result of charge forces between spores and surface. These findings imply that building contamination will likely vary, depending on the specific type of duct material used.  Resuspension rates on both steel and plastic duct materials were between 10^–3 and 10^–5 per second, which decreased to 10 times less than initial rates within 30 minutes. Spores once deposited onto the duct remained a persistent source of contamination over a period of hours.