Lipid-soluble arsenicals, so called arsenolipids, occur in appreciable quantities in various marine organisms including fish. In this study, arsenolipids as well water-soluble arsenic species were investigated in brain (9.1–17.4 mg As per kg; dry mass) and muscle (4.0–5.8 mg As per kg; dry mass) tissues in five specimens of the marine fish skipjack tuna (Katsuwonus pelamis). For this purpose, we developed a sequential extraction method whereby the freeze-dried tissue was first treated with pyridine (organic extract) followed by aqueous ammonium bicarbonate (water extract) to extract arsenolipids and water-soluble species, respectively. When the method was applied to the tuna tissues, the arsenic distribution for brain was 55% (organic extract), 30% (water extract) and 15% (pellet), whereas for muscle tissue the corresponding values were 20%, 55%, and 25%. Arsenic species in water and organic extracts of muscle and brain tissues were investigated by HPLC/mass spectrometry. For both tissues, the water extracts contained arsenobetaine as the major arsenic species together with small amounts of dimethylarsinate and trimethylarsine oxide; trace amounts of dimethylarsinoyl propionic acid were found only in brain tissues. Arsenic-containing hydrocarbons (AsHCs), were the major arsenolipids in both tuna brain and muscle. The arsenolipid content in brain ranged from 3.8–5.9 mg As per kg, whereas it was substantially lower in muscle (0.3–0.8 mg As per kg) reflecting arsenolipids' potential to cross the blood brain barrier and accumulate in the fish brain.