The present research paper deals with the study of the effect of biochemical alterations in DNA content of gill and gonad tissues of the freshwater bivalve Lamellidens marginalis. Samples were collected from the Godavari river at Pravara Sangam district Ahmednagar, Maharashtra. The study of DNA contents of gill and gonad tissues in L. marginalis shows significant decrease in Gill from 4.38 ±0.145 to 3.61 ± 0.246 for 15^th day and on 30th day, there is significant decrease from 3.61 ± 0.451 to 2.86 ± 0.351 whereas Gonad shows significant decrease from 3.87 ±0.246 to 3.21 ±0.126on 15th days and on 30th days there is a significant decrease from 3.54 ±0.369 to 2.52 ±0.214, this is due to toxicity of 5- Fluorouracil and mobilization of tissue in the metabolism.