Abstract: Productivity of an aquatic ecosystem depends on its primary producers. To know how productive a waterlogged paddy field area, the composition, seasonal abundance and dynamics of phytoplankton were assessed from July to November, 2010 covering three main seasons in six different waterlogged paddy fields, Dogi in Laxmipur coastal district, southern Bangladesh. Phytoplankton sample was collected monthly from each Dogi at a depth of 15 cm from the surface using a 1 L glass bottle. The samples were allowed to stand for at least 24 h in the laboratory for the phytoplankton to settle before the supernatant pipetted to concentrate the samples. A total of 64 genera of phytoplankton belonging to 34 families were identified from all sampling locations. Among them 35 genera were found under the class of Chlorophyceae. Commonly occurring taxa were found to be Chlorophyceae, Bacillariophyceae and Cyanophyceae. The phytoplankton density varied from 37384±3656.30 cells L-1 to 319382±14687.05 cells L-1 and the highest being in September and the lowest in July. Statistical analysis showed that there was a significant variation (p≤0.05) in abundance between seasons but not between sampling stations in each separate season. This study provided valuable information for the local fish farmers and aquaculturists that the derelict Dogi’s environment is favorable for practicing aquaculture because of high primary production.