The uptake and fate of the tetramethylarsonium ion was examined in the common shrimp Crangon crangon. Shrimps were fed 10 mg of food (corresponding to $2% of body weight) containing 29, 71, 268 or 580 mg As g Γ1 wet weight in the form of tetramethylarsonium ion daily for 19 days. Two days after the last meal the shrimps were sampled and frozen until dissection. The shrimps were dissected into tail muscle, midgut gland, gills and the remaining tissues ('remainder'), and their total arsenic concentrations were measured by hydride generation-atomic absorption spectrometry (AAS). The arsenic concentration of all four tissues increased with increasing arsenic concentration in the food. The retention efficiency of arsenic in the muscle and 'remainder' tissue was related to the arsenic concentration of the food. The retention efficiency on a whole-animal basis ranged from 29 to 88%, with the highest retention efficiency found at the lowest arsenic concentration of the food. The chemical form of the retained arsenic species was shown to be unchanged tetramethylarsonium ion by cochromatography with standard material using high-performance liquid chromatography (HPLC) with off-line detection by graphite furnace-AAS.