Abstract: Platydorina is a monotypic genus consisting of P.caudata Kofoid found in phyto-plankton samples collected from Shenzhen Reservoir of Guangdong province.The colonies were flattened,slightly twisted in the left sinistral,and horseshoe-shaped with three to five posterior projections in a gelatinous matrix.Each colony contained sixteen or thirty-two cells,a stigma,two contractile vacuoles at the base flagella,and a massive cup-shaped chloroplast with a single basal pyrenoid.Cells were spherical or pear-shaped,arranged in one layer,and oriented in different directions in the gelatinous matrix.Asexual reproduction was accomplished by the repeated division of gonadal cells in the whole or part of the parent colony directly.Sexual reproduction was anisogamous with sperm packets,forming walled hypnozygotes.