Measurements of Terrestrial Gamma Radiation, Primordial Radionuclides and Gross Alpha were made in sediment sample collected from Athangarai Estuary (9^?20'41.99"N - 78'59'55.92"E) in Palk Strait. Water, sediment and selected biota were subjected to 210Po determination. The Terrestrial Gamma radiation level in the estuary was 9.4 ± 3.0m R/h. The Primordial radionuclides levels in the estuarine sediment are ²³⁸U (0.9 Bq/kg), 232Th (8.45 Bqlkg), ⁴⁰K (289 Bq/kg). The Gross Alpha level in Athangarai Estuary was 6.4 m Bq.g^-1. In

water, dissolved concentration of ²¹⁰Po was found to be 1.2 m Bq/l and in sediment (6.3 Bq/kg). Sediment samples of six different sizes grain ranging from 50 µm to 900 pm were subjected to analysis. Particles of 900 µm registered activity between 6.9 Bq/kg and 186 Bqkg. The highest activity of 210Po 6.3 Bq/kg was in the upper layer sediment (0-5cm) and 5.3 Bq/kg at 5-10 cm and 3.8 Bq/kg at 10-15 cm. The ²¹⁰Po activity in the biota was within the range of 12.3 and 345.8 Bq/kg. The bivalve mollusc Meretrix casta of the estuary was identified to concentrate higher level of 2'oPo in their soft tissues, suggesting that it could be used as a sentinel organism for ²¹⁰Po in an estuarine system. The significance of the results are discussed.