The water content in muscle, hepato-somatic index, total protein and osmolarity of hemolymph of red tail shrimp Penaeus penicillatus were examined to understand the chronic effect of mercury toxicity on the growth of the shrimp. The results of the analysis were provided as a reference for pond administration and water quality standardization. The red tail shrimps were exposured in low mercury concentrations for 80 days, their body weight and total length increased with time. The mercury concentration became higher, their growth rate and feed efficiency became lower. The survival rate were 100% after 20 days of exposure in each test group, but survival rates decreased after 80 days in groups with higher mercury concentration. The water content in muscle and hepato-somatic index were significantly related to mercury concentration. The osmolarity of hemolymph were also significantly related to mercury concentration. With the concentration increased, the total protein and osmolarity of hemolymph increased accordingly.