Abstract: Butyltin and phenyltin residues were quantified in Pacific oysters (Crassostrea gigas), which were collected from several aquaculture sites along the west coast of Taiwan during different seasons. Butyltin compounds were detected in all of the samples, except in the oysters collected from the Wanggung site in November 2002. Monophenyltin and diphenyltin compounds were not detected in any of the samples. The average concentrations of monobutyltin, dibutyltin, tributyltin and tetrabutyltin in the oysters were n.d.—406.6±12.7, n.d.—280.9±15.3, n.d.—417.2±11.2 and n.d.—85.8±8.3 ng g−1 (wet wt.), respectively. Concentrations of butyltin compounds found in oysters varied both spatially and temporally.