In the summer of 1990, samplings were made on organisms in and around the eelgrass beds of Koajiro Bay and Moroiso Bay, where the waters were suspected to be severely contaminated by organotins from antifouling paints on recreational boat hulls. The concentrations of tributyltin (TBT), triphenyltin (TPT) and their degradation products, dibutyltin (DBT), monobutyltin (MBT), diphenyltin (DPT), and monophenyltin (MPT), in the samples of some selected organisms were determined to get some insight into their dynamics through coastal organisms including those of eelgrass ecosystems. Concentrations of butyltin compounds in most of animals on the higher trophic levels such as conger eels were lower than those in key industry animals of zooplankton and detritus at the base of a food chain, suggesting that biomagnification through food web did not so intensively operate in the accumulation of butyltins. The pollutant TBT taken up by eelgrass or adsorbed by detritus, on the other hand, was believed to be rapidly degraded to the less toxic MBT by the plant or microorganisms proliferated on the detritus. An inference was also made on the variation in the magnitude of parent pollutant TBT accumulated in the animals and the source of the degradation products, DBT and MBT, in the animals. TPT body burdens were much lower (17 out of 24 samples from Koajiro Bay and 9 out of 19 samples from Moroiso Bay were below detection limit of 1 ng/g wet weight) than TBT body burdens, indicating that the contamination in the study areas was insignificant. However, biomagnification through food web could conceivably play a more hazardous role in the former than in the latter if the contamination progresses in the coastal waters.