In the U.S., ultraviolet (UV) reactors are currently validated using coliphage MS2 as the challenge microbe; however, MS2 is significantly more UV-resistant than Cryptosporidium parvum. The objective of this study was to characterize the UV dose response of microorganisms that better match the dose response of Cryptosporidium than does MS2. The overall approach was to select and acquire potential surrogate microorganisms identified from a thorough literature search and perform bench-scale testing of the prospective surrogates to determine their dose responses to UV irradiation. Of these candidate organisms, only those that demonstrated dose responses similar to C. parvum were further characterized. Methodologies to maximize microorganism yield for large scale propagation (a required characteristic of the virus surrogate) were developed and used to further screen the potential usefulness of the selected organisms. Based on these findings, the surrogate(s) that most closely resembled Cryptosporidium's response to UV was selected for full-scale testing. Includes figure.