After the tragic events of September 11, 2001 in New York and Washington, D.C, the occurrence of a terrorist act involving the use of biological agents is of particular concern to water utilities across the United States. Injection of a relatively small amount of pathogenic agents into a distribution system could impact a large portion of the population served by a water utility. Endospores of Bacillus anthracis (B. anthracis) are among the pathogenic microorganisms of most concern due to their high resistance to chemical disinfectants such as free and combined chlorine. B. anthracis is the microorganism that causes anthrax, a disease that can be 100% fatal when the toxin-mediated infection reaches the systemic form. Inhalation anthrax is more dangerous than the cutaneous or gastrointestinal form. Potential for inhalation related to water could come from exposure to aerosols formed in sinks, showers or sprinkler systems. The U.S. Defense Department has estimated that the lethal inhalation dose (LD50) for humans is 8,000 spores, but studies have shown that doses as low as 9 spores are enough to produce fatalities in 2% of a population exposed to inhalation anthrax. Furthermore, the lack of taste and color of contaminated water with these pathogens plus the relative easiness to produce highly concentrated suspensions make these microorganisms attractive as biological weapons. Consequently, development of strategies for rapid detection and fast response in case of occurrence of such an unfortunate, but realistic event are needed. Based on previous studies, it appears that potential strategies for decontamination could be based on the sequential or simultaneous application of chemical disinfectants. These researchers have shown that limited exposure of C. parvum and B. subtilis to ozone and chlorine dioxide, respectively, increases significantly the rates of inactivation with free and combined chlorine. However, a given combination of these disinfectants producing synergy for C. parvum inactivation might not necessarily result in synergistic effects when used to inactivate B. subtilis spores. Therefore, sequential disinfection schemes have to be evaluated for each microorganism. The main objective of this study was to investigate disinfection strategies for decontamination of a distribution system in the event of a deliberate injection of B. anthracis spores. The non-pathogenic B. subtilis spores were used as a conservative surrogate for B. anthracis spores. Includes 12 references, figures.