The San Francisco Water Department (SFWD) conducted a pilot-scale pathogen seeding investigation with its Hetch Hetchy source water. One purpose of the investigation was to evaluate the removal efficiency of Cryptosporidium oocysts through direct filtration, using anthracite mono-media filters and filtration rates up to 14 gpm/ft2. Treatment consisted of preoxidation with either ozone or chlorine, followed by coagulation and flocculation and filtration. Results indicated that Cryptosporidium removal was equivalent at filtration rates of 6 gpm/ft2. Results also showed that removal efficiency exceeded 2-log during filter ripening, remained between 2 and 3-log removal for most of the filter run, and then declined significantly during particulate breakthrough. In general, particle removal efficiency tended to slightly underestimate Cryptosporidium removal efficiency. Also, when filtered water turbidity was less than 0.1 NTU, Cryptosporidium removal typically exceeded 2-log.