Eutrophication and wave action caused by wind,two important factors determining the phytoplanktonstructures in large shallow lakes. Based on the investigations from October 2003 to September 2004 in Meiliang Bayand Lake Wuli,situated in the northwest of Lake Taihu, the third large shallow lake in China, the spatial distribulion and seasonal variation of environmental variables and phytoplankton community were analyzed to test if trophiclevel and wave action caused by wind were important for phytoplankton community. The results showed that:(1)Meiliang Bay and Lake Wuli were different in trophic states and wave action caused by wind, the former was moreseverely affected by wind and the latter was more eutrophic.(2) The annual mean of the total phytoplankton bio-mass in Lake Wuli (6.85 mgL) was much higher than that in Meiliang Bay (4.99 mgL).The monthly meanreached the maximum in summer and autumn while the minimum in winter in both bays. Seasonal variation of domi-nant phytoplankton assemblages and species in Meiliang Bay was as follows; Bacillariophyta(Cyclotella spp.)andCryptophyta(Cryptomonas app.)in winter, Chlorophyta(Planctonema sp.)in spring, Chlorophyta(genera of Chlorococcales order such as Scenedesmus spp. and Pediastrum spp. et al.),Cyanobacterias (Microcystis spp. andPlanlaothrix app. lt7xillatoria app.),Bacillariophyta (Aulacoxira spp. /Melosira spp.)in summer, Cryptophyta(Cryptomonas spp) in autumn, while the seasonal succession in Iske Wuli was somehow vague. Cryptophyta(Cryptomonas spp.)dominated throughout the year. Sub-dominant or co-dominant species are Bacillariophyta(Aul-acoseira graruilate and Cyclotella spp.)in winter, (:hlorophyta(genera of Chlorococcales order and Cltlamydomonasapp.)in spring, Cyanobacterias (Planktothrex app. and Microcystis app.)and Euglenophyta(Euglena app.)insummer, Baciflariophyta (Aulacoxua app.)in autiunn. 'f1}e difference of phytoplankton community between twobays was resulted mainly from their eutrophic station and wave action caused by wind. (3)Compared with classicalPEG(Plankton Ecology Group) model,the seasonal succession of phytoplankton community in Meiliang Bay andIake Wuli showed some similar pules and they were mainly affected by water temperature, light, nutrient(nitrogenand phosphorus) and the grazing of zooplankton. However, the succession may fluctuate due to the effect of eutrophication and wave action caused by wind. More studies are needed to understand seasonal succession of phytoplankton community in large eutrophic shallow lakes.