Determining the actual inactivation efficiency of Cryptosporidium parvumachieved in water treatment plants is not currently feasible due largely to the fact that current viability assessment techniques do not work at the low levels at which oocysts of this pathogen occur in natural waters. The use of surrogate indicators for C. parvum has the potential to help overcome this impasse. Both biological and non-biological indicators have been proposed for use as a surrogate for C. parvum. Promising biological surrogate indicators for C. parvum are aerobic spore-forming bacteria. In particular, Bacillus subtilis spores are good candidates because they are non-pathogenic and relatively easy to culture and assess for viability. The objective of this study was to investigate how accurately the inactivation kinetics of Bacillus subtilis spores matched those of C. parvum oocysts using two chemical disinfectants, molecular ozone and monochloramine, independently and sequentially under pH, temperature, and concentration conditions found in water treatment facilities. Includes 9 references, figures.