In accordance with the Agricultural Bioterrorism Protection Act of 2002, we are amending and republishing the list of select agents and toxins that have the potential to pose a severe threat to animal or plant health, or to animal or plant products. The Act requires the biennial review and republication of the list of select agents and toxins and the revision of the list as necessary. This action implements the findings of the third biennial review of the list. In addition, we are reorganizing the list of select agents and toxins based on the relative potential of each select agent or toxin to be misused to adversely affect human, plant, or animal health. Such tiering of the list allows for the optimization of security measures for those select agents or toxins that present the greatest risk of deliberate misuse with the most significant potential for mass casualties or devastating effects to the economy, critical infrastructure, or public confidence. We are also making a number of amendments to the regulations, including the addition of definitions and clarification of language concerning security, training, biosafety, biocontainment, and incident response. These changes will increase the usability of the select agent regulations as well as provide for enhanced program oversight.