Arsenic levels in the sea weed, Hijiki, Sargassum fusiforme (Harvey) Setchell, after soaking in water were determined by thermal neutron activation analysis. Commercial dried Hijiki was soaked in purified water for periods of time (20-360 min) at various temperatures (0-90°C). Arsenic concentrations retained in the swollen Hijiki tissues and those dissolved in the aqueous solution were determined. At temperatures higher than 30°C, about 70% of the total arsenic was removed from the swollen Hijiki after discarding the soaking water. The non-extractable arsenic bound to the tissues seems to be about 10% of the total, when estimated at higher temperatures (75 and 90°C). To remove arsenic as much as possible before cooking, it is recommended to soak commercial dried Hijiki in warm water for more than 30 min and discard the water extracts.