Physiochemical studies on water and sediment quality and plankton diversity of Posna beel, Kalihati, Tangail were conducted during 2003-2005. Surface water temperature was minimum (16°C) during winter and maximum (33 °C) during summer. Water transparency varied from 25 cm in summer to 189 cm in winter. Other water quality parameters like pH (7.2-8.0), DO (6.6-9.6 mg/L), free CO2 (5-13 mg/L), alkalinity (28-51 mg/L), total solids (7-123 mg/L), BOD ((2.70-5.43 mg/L), COD (5.20-8.15 mg/L), total hardness (51-68 mg/L), NH3 (0.01-0.07 mg/L), NO3-N (0.15-0.48 mg/L) were found to be suitable for freshwater fishes. The beel sediment was rich in nutrient and contained a high amount of organic matter (3.84-6.25 %). Some essential compounds like iron (417-440 µg/g), manganese (23.93-55.63 µg/g), copper (8.0-11.63 µg/g), magnesium (1.83-2.56 meq/100 g) were also found to be high. A total of 48 genera of phytoplankton and 41 genera of zooplankton were recorded. Dinoflagellate bloom was observed throughout the study period. Shannon-Weiner biodiversity indices were found to be higher during monsoon for zooplankton while phytoplankton abundances showed no such specific relationship with season. The physicochemical and biological parameters indicate that the beel was eutrophic in nature.