Safe disposal of dead livestock and contaminated manure is essential for the effective control of infectious disease outbreaks. Composting has been shown to be an effective method of disposal, but no information exists on its ability to contain diseases caused by spore-forming bacteria, such as Bacillus anthracis. Duplicate composters (east and west), each containing 16 dead cattle, were constructed (final capacity, 85,000 kg). Spores (10(7) CFU/g manure) of Bacillus licheniformis and Bacillus thuringiensis were mixed with autoclaved feedlot manure and placed in either sterile vials or porous nylon bags. Compost temperatures in the west composter were slightly higher than in the east composter. Viable B. thuringiensis spores were reduced to ≤10(2) CFU in all samples after 112 days but were isolated from bags (west composter) at ≤10(2) and at 10(5) CFU (east composter) after 230 days. In contrast, B. licheniformis was at ≤10(2) CFU in vials (west composter) after 112 days but remained at 10(6) CFU after 230 days (east composter). Similarly, B. licheniformis in bags was not detected after 230 days in the west composter but remained at 10(7) CFU in the east composter. Our study suggests that spore viability was reduced in the west composter by exposure to compost and elevated temperatures over time. Different temperature profiles may explain why spores remained viable in the east structure but were largely rendered nonviable in the west structure. Under practical conditions, variation in composting microclimates may preclude the complete inactivation of Bacillus spores, including those of B. anthracis, during composting. However, composting may still have merit as a method of biocontainment, reducing and diluting the transfer of infectious spores into the environment.