The occurrence of imposex in Adelomelon brasiliana during two consecutive years near to Mar del Plata (the principal fishery port of Argentina) and its relation to tributyltin (TBT) and dibutyltin (DBT) content in muscle tissue and digestive gland was investigated using gas chromatography. Percentage of imposex affected females increased to over 80% in spring and summer of both years. However, its intensity, given by the relative penis size index (RPSI), fluctuated around low values. Adelomelon brasiliana digestive gland samples showed a very low concentration of TBT (36–46 ng/g dry weight). Egg capsules presented normal viable embryos, which had TBT concentrations similar to the capsule wall (15 ng/g) while the intracapsular liquid was only 2 ng/g, similar to the sediment concentration. The proportion and distribution of butyltin compounds (BTs) in sediments indicate a major contamination event.