Abstract: In order to understand the variation of planktonic microalgae in low salinity culture ponds of Litopenaeus vannamei, investigations on the planktonic microalgae species and quantity were conducted from September 2007 to January 2008. The results showed that a total of 49 species of microalgae was identified in the culture ponds, belonging to 7 phyla and 62 genera, respectively. They included 22 Chlorophyta spcies, 16 Cyanophyta species, 8 Bacillariophyta species, 2 Euglenophyta species, and 1 Cryptophyta species. Many of the 13 dominant species belonged to the Cyanophyta, including Microcystis aeruginosa, Coelosphaerium naegelianum, Oscillatoria chlorine, Pseudoanabaena sp., Spirulina spirulinoides, O.subbrevis, Chroococcus cohaerens, Merismopedia punctata and Dactylococcopsis aciculari. The other dominant species belonged to Chlorophyta and Bacillariophyta, including Chlorella pyrenoidosa, Chlamydomonas.multgranulis, Chaetoceros sp. and Nitzschia closterium. In low salinity culture ponds, the quantity of planktonic microalage ranged from 0.1×10⁷~209.2×10⁷ ind · L^-1, and diversity index averages were at 2.5~3.2. The species numbers, quantity and diversity index were low in early culture period and high in post culture period.