Studies on the changes in the biochemical constituents and calorific values have been carried out in the estuarine clam Katelysia opima (Gmelin) from the Vellar Estuary for the period of one year. Protein, carbohydrate and lipid were very low during spawning period and high in maturation period, the values are slightly higher in females than males. Mean calorific value found to be high in female than in males. Mean calorific value found to be high in female than in male. Change in the biochemical components are mainly due to the reproductive cycle and also by food availability.