Ultraviolet light has been shown to be an effective disinfectant for protozoan cysts and the US Environmental Protection Agency is presently establishing dose requirements for potable water applications. However, much of the work establishing UV dose-response performance was obtained using filtered potable water or laboratory-grade water of low turbidity. To address the performance of this technology for unfiltered supplies, the Los Angeles Department of Water and Power, five unfiltered water agencies, and MWH (Montgomery Watson Harza) investigated the inactivation of Cryptosporidium parvum and Giardia muris in raw water supplies with turbidity values ranging from 0.65 to 7 NTU. A series of bench-scale collimated beam experiments and a six month pilot-scale study were conducted to investigate the ability of UV light to inactivate Giardia muris and Cryptosporidium parvum under a variety of water quality and operational conditions. Includes reference, tables.