Aim: To measure the decline rates of zoonotic agents introduced into liquid livestock wastes in on- farm storage tanks. Methods and Results: Salmonella spp., Escherichia coli O157, Campylobacter jejuni, Listeria monocytogenes and Cryptosporidium parvum, propagated in laboratory- controlled conditions, were inoculated into 35 000- l volumes of fresh livestock wastes ( pig slurries, cattle slurries and dirty waters). D- values for bacteria were six to 44 days, and for C. parvum were 133 to 345 days. Campylobacter jejuni declined significantly more rapidly than the other bacterial pathogens, while E. coli O157 declined significantly more slowly. On average, bacterial declines were not affected by the season of waste deposition and storage or by the dry matter content of the wastes, but were more rapid in dirty waters than in pig slurries. The physiciochemical composition of wastes in each category varied significantly. Conclusions: Zoonotic agents can survive for several months during storage of liquid livestock wastes. Livestock wastes should be batch-stored and not subjected to continuous additions. Significance and Impact of the Study: This study indicates that batches of liquid livestock waste, if contaminated with bacterial pathogens, should be stored for 6 months to reduce contamination levels. Alternative strategies for reducing C. parvum levels in liquid livestock wastes should be explored.