Woronina polycystis Cornu as a parasite of water molds ( Saprolegnia diclina ) is illustrated and described as new to Taiwan. It is morphologically characterized by its plas-modia, zoosporangial sori with zoospores in abundance and exit tubes as well as cystosori with resting spores 'fill-ing the host cells. This fungus belongs to the Family Plas-modiophoraceae in the Order Plasmodiophorales.