Current methods to concentrate viruses and Cryptosporidium parvum oocysts from large volumes of water are prone to inconsistent results and are costly and complex. In addition, the recovery of viruses and oocysts requires the use of two different concentration methods. Ultrafiltration utilizes size selection rather than adsorption/elution to concentrate any organisms larger than the pore size of the ultrafilter. This approach in the concentration of pathogens from water may offer greater flexibility in developing methods that can provide more consistent recoveries among different viruses and widely varying water conditions. This study involved the characterization and optimization experiments were done using two small-scale (2L) and two large-scale ultrafiltration (100L) systems (hollow fiber and tangential flow) with virus suspended in reagent grade water, tap, ground, and surface waters. Recovery experiments were done with three viruses: bacteriophage PP7 and T1 and poliovirus as well as a protozoan parasite (C. parvum oocysts) to compare, characterize and optimize the recovery with two ultrafiltration systems. Pretreatment of the ultrafilters with blocking agents (fetal bovine serum or other proteinaeous solutions) and the use of elution agents can serve to prevent viral adsorption to the filter surface or to elute bound virus and keep viral agents suspended in the retentate. Blocking the membrane also improved C. parvum recovery. The use of a blocking and an elution step efficiently concentrated (>60% recovery) viruses and C. parvum oocysts from widely varying water qualities including surface water. Both ultrafiltration systems appear to be able to recover viruses efficiently; however, the hollow fiber systems may provide slightly better and more consistent results in the 2 and 100L volumes tested. These results indicate that the hollow fiber ultrafiltration system can efficiently and reproducibly recover viruses and C. parvum from small- and large-scale systems and from widely varying water qualities, and that both ultrafiltration systems can provide efficient recovery of viruses from water.