Ostracods are tiny crustaceans found in aquatic habitats and the present paper deals with the role of the water quality index on their population diversity and seasonal fluctuations at the three different sampling stations of Cauvery River, Tiruchirappalli (Tamil Nadu, South India) on a regular biweekly basis from September 2019 to August 2020. The highest level of a hydrological parameter, atmospheric temperature 33.75±0.353°C on April-20 at S-II; water temperature 32.00±0.353°C on June-20 at S-III; pH 8.5±0.106 on December-20 at S-II; DO 8.78±0.155 mg/l on December 20 at S-III; salinity 0.996±0.000 on March at S-I; phosphate 2.38±0.304 mg/l on August 20 at S-III; turbidity 3.8±0.070 NTU on February 20 at S-III; total solids 681.0±7.778 mg/l on May 20 at S-III; total hardness 246.0±4.242 mg/l in September 2029 at S-III and diversity of Ostracods population [533.5±6.71 ind/l] in S-III; [521.5±6.01 ind/l] in S-II and [510.0±2.82 ind/l] in S-I were observed in May 2020. Thus, ostracod aggre-gations exhibit favorable hydrological conditions and the response offers the possibility of use as pollution indicators for the Cauvery River Tiruchirappalli.