Abstract: Three species and one variety of cryptomonad flagellates belonging to three genera were observed with the scanning electron microscope in sea water samples from Tolo Harbor (Hong Kong), Changjiang River Estuary and Xiamen Harbor. They are Hemiselmis sp. Novarino, Plagioselmis prolonga Butcher ex Novarino, Lucas & Morrall, Plagioselmis prolonga var. nordica Novarino, Lucas & Morrall and Teleaulax acuta (Butcher) Hill. The taxonomic characteristics, ecological habit and distribution of the above species are described and the LM and SEM photographs of the species are also presented. This is the first record of the genus Hemiselmis Parke in the China Sea, and the species Plagioselmis prolonga and Teleaulax acuta have records of producing blooms in the China Sea.