Fluctiation Asymmetry (FA) as the deviation from the symmetry of each individual has been used as an indicator of genetic quality of individual or environmental quality of the habitat. The environmental factors that can influence the development of animals include physical and chemical factors such as food availability, stresses, and disturbance. As stresses increase, FA is expected to increase. It is because there is an energy trade-off between maintaining the accuracy of development and recovering from stresses. Sentinel crabs Macrophtalmus japonicus and M. banzai are widely distributed all over the typical mudflats of Korea. We are measuring FA of these crabs to determine if FA is dependent on the heavy metal pollution of Korean mudflat.  We would apply FA of crabs to evaluating the environmental condition of mudflats.