A reagent and method for the specific and highly sensitive detection of C. parvum in which the reagent is an antibody for a soluble C. parvum sporozoite antigen and the method is an immunoassay in which the antibody is used to detect or quantify C. parvum sporozoite antigen in a sample. The sample is treated to cause excystation of C. parvum oocytes, thereby releasing a C. parvum sporozoite antigen, and combined with antibodies specific for the sporozoite antigen under conditions to form an antibody-antigen complex. Detection of the complex indicates the presence of C. parvum in the sample. The assay allows recognition and detection of C. parvum in turbid samples, and due to a lack of crossreactivity with other Cryptosporidium species, is specific for C. parvum contamination or infection. The assay is highly sensitive, allowing for the detection of less than 100 oocysts per milliliter of sample.