The main objective of this study was to assess the effect of temperature (4-30 degrees C) on the inactivation kinetics of Cryptosporidium parvum oocysts with sequential disinfection schemes involving the use of chlorine dioxide as the primary disinfectant and free or combined chlorine as the secondary disinfectant in synthetic water. The synergy previously reported for sequential inactivation of C. parvum oocysts with ozone/free chlorine or ozone/combined chlorine did not occur when chlorine dioxide was used. instead of ozone, as the primary disinfectant within the temperature range (4-30 degrees C) and the pre-treatment levels investigated. Sequential ozone/chlorine dioxide and chlorine dioxide ozone experiments revealed that the lower level or absence of synergy for chlorine dioxide/free chlorine and chlorine dioxide, monochloramine was likely the result of chlorine dioxide reacting with oocyst chemical groups that are mostly different from those reacting with ozone, free chlorine, or monochloramine. The CT concept was found to be valid for the primary inactivation kinetics of C. parvum oocysts with chlorine dioxide, thus allowing the use of the simpler CT approach for the development of C. partum inactivation requirements with chlorine dioxide. General consistency was found between the secondary inactivation kinetics of C. parvum oocysts with free chlorine and monochloramine after chlorine dioxide pretreatment obtained in this study with oocyst viability determined by a modified in vitro excystation method and those reported in the literature for the same sequential disinfection schemes based on an animal infectivity assay.