The inactivation of Cryptosporidium parvum oocysts by chlorine dioxide is pH dependent with better inactivation occurring at higher pH. Different lots of oocysts have different resistance to inactivation. This variable resistance to inactivation suggests that care must be taken in utilizing inactivation data for design purposes. The in vivo mouse infectivity assay demonstrates a higher log inactivation than either the in vitro cell culture infectivity or in vitro modified excystation assays. Includes 11 references, tables, figures.