The application of chemical disinfectants sequentially have been shown to provide superior disinfection performance for encysted parasites when compared to the same disinfectants used singly. This finding may be of great importance in setting new regulations under the Enhanced Surface Water treatment rule. Inactivation of Giardia muris cysts, and Cryptosporidium parvum oocysts were studied at bench-scale using ozone or chlorine dioxide in combination with chlorine species. The findings suggest that for both Giardia and Cryptosporidium, a 0.5 to 1 log-unit benefit may be possible under the conditions studied when compared to the use of the disinfectants singly. The use of combinations of disinfectants may provide a practical, cost effective method for water utilities to achieve control of waterborne parasites. Further research is necessary to develop design criteria.