Available literature data indicate that Cryptosporidium oocysts are removed more efficiently than aerobic spore forming bacteria during water treatment processes involving clarification and filtration. Therefore, monitoring removal of naturally occurring aerobic spores can provide a conservative estimate of the Cryptosporidium removal in full-scale water treatment processes, which in turn can be used to establish the demonstration of performance (DOP) credit allowed by the LT2ESWTR, as well as providing a valuable performance evaluation tool for water plant operators. Results from this study showed that: median raw water levels ranged from 300 to 3,000 spores/L for two lake sources and 80,000 to 400,000 spores/L in four river sources; median filtered water spore levels rarely exceeded 5 spores/L and these levels of removal were not apparently dependent on source water concentration; and, >4-log spore removal could not be demonstrated unless raw water levels exceeded 10,000 spores/L (e.g., river sources). Includes 12 references, table, figures.