The purpose of this study is to establish the background metal concentrations of several demersal aquatic organisms in the coastal waters off Yunlin county before the development of petroleum industries camp at Tai-shi area, S. W. Taiwan. In addition to, the proper bioindicators are also investigated for future monitoring purpose. Twenty-seven species of sedimentary organisms, including 11 species of fishes, 3 species of crabs, 9 species of shrimps, 2 species of mollusca, and 2 species of cephalopods were collected in 5 seasons from September 1998 to November 1999. Metal concentrations of Ag, As, Cd, Co, Cu, Fe, Hg, Mn, Se and Zn were analyzed in the muscles and internal organs. The results showed that metal concentrations were significantly different according to species and tissues. Muscle concentrations of As, Fe and Zn were higher in fish, shrimp and cephalopod, whereas Zn, Cu and As were in crab, and Fe and Zn in mollusca. In the internal organs, Fe concentrations were the highest metal contents of fish and mollusca, but Cu were the highest for crab and shrimp. Nevertheless, Fe and Cu concentrations were respectively the highest elemental contents in two species of cephalopods, Octopus ocellatus and Sepiotenthis lessoniana. In gonads, Zn was the highest metal concentration in both gender of fish. However, in crab, Cu and Zn were respectively the highest metal concentrations of Portunus pelagicus and Charybdis hellerii. Among the all, Ag, Co, Cd and Hg showed the lowest concentrations in the species. In most cases, the metal concentrations of internal organs were higher than those of muscles, except Ag and Hg turned out to be reverse. The mean metal concentrations found in this study were similar to those measured in uncontaminated waters worldwide, so that it represents the background values of the aquatic demersal organisms in Yin-lin waters. The metal concentrations in the editable parts of the organisms were all below the food safety standard worldwide. Among the 10 metals analyzed, only As, Se and Zn concentrations of gonad showed the gender-differences. A positive correlation between a metal concentration and the body size were found in several cases, e. g. Ag in internal organs of Neverita lineata, As in muscles of Penaeus cornutus, Hg in muscles of Sillago sihama, Penaeus cornutus, Penaeus hardwickii and Neverita didyma, and Zn in hepatopancrea of Portunus pelagicus. However, Zn in viscera of Neverita didyma revealed a negative correlation to the size. Seasonal variations of metal concentrations were also found in the internal organs of most species. The concentrations of Cu, Fe, Mn, Se, Zn and As of internal organs showed a peak in both February and April, which may relate to the seasonal in put of anthropogenic pollutants in the area. Based on the metal concentrations of internal organs, the ratio of internal organ to muscle and the relative deviations, we suggest Octopus ocellatus, Portunus sanguinolentus (great for Ag, Cd, Co, Cu, Se and Zn), Penaeus cornutus (great for As and Cd), Neverita didyma(great for Co, Hg and Zn) and Arius maculatus(great for Fe, Se and Zn), which potentially concentrated high levels of metals, could be the good bioindicators in future.