The objective of this study was to investigate the synergy involved in ozone/free chlorine and ozone/monochloramine sequential disinfection schemes. The ozone/free chlorine disinfection scheme was reported previously; thus, the main focus of this paper is the ozone/monochloramine sequential disinfection scheme. In this study, monochloramine was formed with approximately 6.4 mg/L as N excess ammonia; thus, the effect of ammonia after pretreatment with ozone was also investigated. Oocyst viability was determined by a modified in-vitro excystation method that is consistent with animal infectivity data. Includes 15 references, figures.