Ammocoetes of sea lamprey (Petromyzon marinus Linnaeus, 1758) from an assumedly pristine tidal freshwater habitat were used for determination of metal levels and accumulation patterns with size. The individuals were collected during the winter of 2011 in river Minho (Iberian Peninsula, Southwest of Europe) and the levels of metals (Cd, Cu, Cr, Fe, Pb, Mn and Zn) in the whole body were analysed by atomic absorption spectrometry. Fe was the metal with highest levels (maximum of 1438 ppm), while Cd and Pb were the lowest accumulated elements (overall means of 0.093±0.065 ppm and 0.259±0.094 ppm, respectively). Larger ammocoetes seemed prone to accumulate Cu, Fe and Mn (P<0.05). Pb and Zn levels also seemed to increase with ammocoetes size; however, statistical differences were not found between size classes (P>0.05).