Traditionally, low-pressure (LP) ultraviolet (UV) lamps, providing nearmonochromatic UV light at 254 nm have been the standard for use in UV disinfection. Recently, polychromatic UV systems such as pulsed-UV and medium-pressure (MP) UV were found to be effective for the inactivation of the persistent pathogen Cryptosporidium parvum. More recently, it has become apparent that monochromatic low-pressure UV is also very effective for the inactivation of Cryptosporidium. In fact, it appears that this organism is more susceptible to disinfection by UV light than are most enteric bacteria. The question still remains, however, whether there is an advantage to the use of lampsthat produce polychromatic UV over those that emit monochromatic UV. To assess the relative effectiveness of polychromatic and monochromatic UV radiation for the inactivation of waterborne microorganisms, experiments were performed to evaluate the efficacy of distinct bands of UV light for the inactivation of Cryptosporidium parvum oocysts and MS-2 coliphage. Includes 7 references, table, figures.