The protozoan parasite, Cryptosporidium parvum, continues to pose public healthproblems throughout the United States. Studies have shown that Cryptosporidiumoocysts persist in the environment and are resistant to disinfection practicescommonly used for drinking water treatment. However, some of these previousstudies employed several commonly used Cryptosporidium viability assays that areunreliable predictors of the infectivity of oocysts subjected to disinfectants orenvironmental stresses. Tissue culture methods that measure infectivity haveproven to be more reliable than either vital dye staining or in vitro excystationassays. In this study, the persistence of C. parvum oocysts in environmentalwaters was studied using tissue culture infectivity and viability assays. Oocystswere suspended in three representative environmental water samples: a surfacewater of relatively low turbidity; a surface water of relatively high turbidity;and a seawater sample. In order to evaluate the effect of temperature on oocystviability, each sample was tested at low (4 degrees C) and moderate (25 degreesC) temperatures. Includes 16 references, tables, figures.