The Surface Water Treatment Rule (SWTR) requires a three log (99.9 percent) removal/inactivation of Giardia and a four log (99.99 percent) removal/inactivation of viruses. The Enhanced SWTR may require some utilities to provide even greater removal of these organisms, depending on source water quality. As a result of the regulations and in anticipation of a Groundwater Disinfection Rule, there has been an increasing interest in employing low pressure membrane filtration for control of microorganisms from untreated ground and surface waters. Further, Cryptosporidium parvum (C. parvum) is becoming an organism of increasing concern to the water industry; to date there is no information in the peer-reviewed literature on the efficacy of membrane technology for removal of this protozoan oocyst. The objective of the investigation was to evaluate the efficacy of various ultrafiltration (UF) and microfiltration (MF) membranes in removing C. parvum, Giardia muris (G. muris), viruses and other microbial indicators from synthetic, surface, and groundwaters. This study is part of a larger project funded by the American Water Works Association Research Foundation (AWWARF) on the use of low pressure membrane technology for water treatment.