Cryptosporidium is a coccidian protozoan and has recently been recognized as a cause of human gastroenteritis. The illness may be severe in immunocom-promised individuals and can result in death. The organism is monoxenous, and infective oocysts are shed in the feces, which are transmitted via the fecal-oral route. Therefore, Cryptosporidium may be a possible candidate as an etiological agent of waterborne or foodborne disease. Previously, the lack of methods for detection of this organism in diarrheal samples as well as environmental samples, has made it difficult to substantiate this premise. Detection of Cryptosporidium oocysts in water has now been accomplished using a filtration-elution procedure. A fluorescently-labelled monoclonal antibody which has been used for the detection of Cryptospori di urn in feces was also used to detect the oocysts in environmental samples. The method developed for detection of Cryptosporidium in water including: 1) filtration with polypropylene cartridge filters, 2) elution with Tween 80/detergent, 3) concentration with Sheather's flotation, and 4) detection on membrane filters, was evaluated and used to examine wastewater and fresh water for Cryptosporidiurn oocyst concentrations. Recovery from the sucrose gradient step averaged 72 – 82%. Overall recovery efficiencies ranged from 25 – 82% (average 50%), but were diminished with low seeds. Concentrations were found in ranges from 60 to 52,000 oocysts/gal in treated and raw sewage. In fresh water, oocysts numbered 8 to 22,000/gal and were concentrated in rapid sand filters averaging 59,000/gal in the backflush. Cryptosporidium may now be considered a potential waterborne parasite.