Between September 1986 and May 1988 a study of an operating slow sand water filtration plant was undertaken to determine its efficiency in removing Giardia and Cryptosporidium from a source water. The limited quantified data demonstrated an average 93 percent removal of Giardia cysts by the filtration plant, with this protozoan being detected in only one of 35 unfiltered water samples. An average 48 percent of Cryptosporidium oocysts was removed. Cryptosporidium was detected in 46 percent of the filtered samples. These findings suggest that Cryptosporidium may not be adequately removed from a contaminated source water under very cold operating conditions or if the filtration plant does not comply with accepted design standards. Other variables were turbidity and total and fecal coliform bacteria. Includes 40 references, tables.