The total protein increased in the gills and decreased in the muscle of the freshwater field crab Oziotelphusa senex senex at days 1 and 2 on exposure to lethal concentrations and at days 1 and 10 to sublethal concentrations of furadan, endosulfan, chlorpyrifos, and a mixture of these three in a 100:10: 1 ratio. The increase in the gill protein was greater on exposure to the sublethal concentrations than to the lethal concentrations while the decrease in the muscle protein was greater on exposure to the lethal concentrations than to the sublethal concentrations. In the hepatopancreas, the protein content decreased on exposure to the lethal concentrations, but, in contrast, increased on exposure to the sublethal concentrations. These results clearly indicate that changes in the protein content are not only organ-dependent but also concentration-dependent, i.e., lethal versus sublethal. Irrespective of the changes in the total protein, the levels of free amino acids and the activities of protease, alanine and aspartate aminotransferases, and glutamate dehydrogenase increased in all the three organs of the crabs exposed to the lethal and sublethal concentrations, (more in lethal than in sublethal) and increased at a greater rate over time of exposure. Ammonia toxicity, measured by an increase in the hemolymph ammonia and a decrease in the urea, was also observed at the lethal concentrations of all the three pesticides. The ammonia and urea levels increased in the crabs exposed to the sublethal concentrations. Although the effect of each pesticide on the protein metabolism was similar, the degree of toxicity was the lowest on exposure to furadan, intermediate on exposure to endosulfan and chlorpyrifos, and cumulative on exposure to a mixture of the three pesticides The total protein increased in the gills and decreased in the muscle of the freshwater field crab Oziotelphus senex senex at days 1 and 2 on exposure to lethal concentrations and at days 1 and 10 to sublethal concentrations of furadan, endosulfan, chlorpyrifos, and a mixture of The three in a 100: 10: 1 ratio. The increase in the gill protein was greater on exposure to the sublethal concentrations than to the lethal concentrations while the decrease in the muscle protein was greater on exposure to the lethal concentrations than to the sublethal concentrations In the hepatopancreas, the protein content decreased on exposure to the lethal concentrations, but, in contrast, increased on exposure to the sublethal concentrations. These results clearly indicate that changes in the protein content are not only organ-dependent but also concentration- dependent , ie, lethal versus sublethal. Irrespective of the changes in the total protein, the levels of free amino acids and the activities of protease, alanine and aspartate aminotransferases, and glutamate dehydrogenase increased in all the three organs of the crabs exposed to the lethal and sublethal concentrations, (more in lethal than in sublethal), and increased at a greater rate over time of exposure. Ammonia toxicity, measured by an increase in the hemolymph ammonia and a decrease in the urea, was also observed at the lethal concentrations of all the three pesticides. The ammonia and urea levels increased in the crabs exposed to the sublethal concentrations. protein metabolism was similar, the degree of toxicity was the lowest on exposure to furadan, intermediate on exposure to endosulfan and chlorpyrifos, and cumulative on exposure to a mixture of the three pesticides