1. The changes in the biochemical composition of gills, hepatopancreas and muscle after exposure to 0.25 mg Hg/l were studied in Procambarus clarkii. 2. Sublethal exposure to mercury in P. clarkii resulted in significant decreases in protein concentration and caloric concentration in gills over the 96 hr period. Glycogen/lipid and glycogen/protein ratios increased after 48 and 96 hr of mercury exposure. 3. Lipid and caloric concentration in the hepatopancreas were significantly lower in 96 hr mercury exposed group. 4. Glycogen concentration in muscle was depleted as consequence of 96 hr mercury exposure.