Cadmium (Cd), one of the twenty three heavy metal toxicants, is widely used in Ni-Cd batteries manufacture, metal and mining industry, dentistry etc. because of its non-corrosive nature. Cd is released in considerable amounts through industrial effluents into soil, surface and ground water systems. These excess amounts in addition to naturally occurring levels gradually build up to toxic levels causing damage to the biota of the aquatic ecosystem. It shows biomagnification and has greater half-life periods. Cd was found to interfere with many protein and carbohydrate metabolisms by inhibiting the enzymes involved in the processes. The present study evaluates toxicity of Cd and its impact on biochemical constituents like glucose, glycogen, total proteins, lipid and free aminoacids in the fresh water edible carp Catla catla as Cd bioaccumulation can affect humans through biomagnification. Short term tests of acute toxicity were performed over a period of 96 hours using cadmium chloride. The renewal technique was followed by exposing the fish to test solutions of different concentrations in the range of 1 mg/L to 8 mg/L. Preliminary experiments were conducted to choose concentrations that resulted in the mortality of the fish in the range of 10-90%. The toxicity experiments were then conducted using the chosen concentrations of CdCl2 on the fingerlings ( Wt. 6±1 grams ) in triplicate and the LC50 was determined using simple graphic ( % Mortality Vs. Log Concentration), probit graphic ( Probit value Vs. Log concentration) and unweighted regression analysis methods. The calculated average 96-hr LC50 is 4.533 mg/L and the equation for the dose-mortality regression line was found to be Y = 2.65X + 3.368. Levels of the five biochemical constituents viz., Glucose, Glycogen, Total proteins, Lipids and Free aminoacids were determined by standard biochemical procedures in the five tissues i.e., muscle, gill, liver, heart and kidney of unexposed (Control) healthy fish and the fish exposed to 96-hr LC50 (Lethal) and sub-lethal concentrations (1/10th of the lethal dose for 7 days of exposure) of cadmium chloride. Results showed significant fall in all the biochemical constituents in all the tissues except glucose prompting to suggest that the fish cultured in the aquatic systems closer to the industrial locations would not have the expected nutritive value. The elevated levels of glucose are apparently indicative of the organism's response to the toxicant stress. Also, such fish when consumed as food lead to the deposition of the heavy metal in the soft tissues of the human body leading to exposure health effects. Key words: Toxicity; Heavy metal; Cadmium chloride; Biomagnification; Catla catla DOI: 10.3126/kuset.v3i2.2890  Kathmandu University Journal of Science, Engineering and Technology Vol.3, No.2, August 2007, pp 1-11