The scallop cultivating sea area in Qinhuangdao,Hebei Province is one of the main cultivating areas in China.The scallop cultivation has facilitated the eutrophication in this area and made the biological resources extremely rich in coastal waters off Qinhuangdao,especially the phytoplankton.Through a two-year sampling investigation in different seasons on phytoplankton at 4 sites,the species diversity,ecological and seasonal distribution of phytoplankton in coastal waters off Qinhuangdao was studied.A total of 91 species belonging to 45 genera in 5 phyla were identified.The following phyla were represented: Bacillariophyta(69 species in 30 genera),Pyrroptata(18 species in 11 genera),Chrysophyta(1 species in 1 genus),Chlorophyta(1 species in 1 genus)and Euglenophyta(2 species in 2 genera).The seasonal occurrence of phytoplankton was studied as well.Both of the phytoplankton cell density and species in summer were most compared to that in winter or spring.Diatoms were the dominant species all year except on April,when Protoperidinium pellucidum(dinoflagellate)dominated phytoplankton.The dominant phytoplankton species was Protoperidinium pellucidum and Rhizosolenia stolterforthii in spring,Nitzschia sp.and Chaetoceros spp.in summer,and Cerataulina pelagica and Ditylum brightwellii in winter respectively.