The U.S. National Research Council (NRC) concluded in 2002 that U.S.agriculture is vulnerable to attack and that the country has inadequate plansfor dealing with agricultural bioterrorism. This article addresses thevulnerability of U.S. crops to attack from biological weapons by reviewing thecosts and impact of plant diseases on crops, pointing out the difficulty inpreventing deliberate introduction of pathogens and discovering new diseaseoutbreaks quickly, and discussing why a plant pathogen might be chosen as abiological weapon. To put the threat into context, a brief historical review ofanti-crop biological weapons programs is given. The argument is made that thecountry can become much better prepared to counter bioterrorism by developing alist of likely anti-crop threat agents, or categories of agents, that is basedon a formal risk analysis; making structural changes to the plant protectionsystem, such as expanding diagnostic laboratories, networking the laboratoriesin a national system, and educating first responders; and by increasing ourunderstanding of the molecular biology and epidemiology of threat agents, whichcould lead to improved disease control, faster and more sensitive diagnosticmethods, and predictions of disease invasion, persistence, and spread followingpathogen introduction.