This paper summarizes a four-year study conducted at Orange County Water District's Water Factory 21 in California, in which enteroviruses were monitored. Water Factory 21 is a 56-ML/d advanced wastewater treatment plant. The process train included high pH-lime treatment, air stripping, recarbonation, prechlorination, multimedia filtration, granular activated carbon adsorption, reverse osmosis demineralization, and final chlorination. The influent and effluent were sampled on the average of once per week. Viruses were detected twice during the four-year period in chlorinated effluents at very low levels and were associated with either a typical plant operations, combined chlorine residuals, or high turbidity. Plant operating criteria were then modified to include use of downflow granular activated carbon, manual addition of chlorine, and the change in the source of the plant influent. Since these changes have been implemented, no positive samples of enteroviruses have been identified. Includes 9 references, tables, figures.