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To determine the ability of duckweed ponds used to treat domestic waste-water to remove Giardia and Cryptosporidium.The influent and effluent of a pond covered with duckweed with a 6 day retention time was tested for Giardia cysts, Cryptosporidium oocysts, faecal coliforms and coliphage. Giardia cysts and Cryptosporidium oocysts were reduced by 98 and 89%, respectively, total coliforms by 61%, faecal coliforms by 62% and coliphage by 40%. There was a significant correlation between the removal of Giardia cysts and Cryptospordium oocysts by the pond (P < 0·001). Influent turbidity and parasite removal were also significantly correlated (Cryptosporidium and turbidity, P =0·05;Giardia and turbidity, P =0·01).The larger organisms (parasites) probably settled to the bottom of the pond, while removal of smaller bacteria and coliphages in the pond was not as effective.Duckweed ponds may play an important role in wetland systems for reduction of Giardia and Cryptosporidium.