Generally, heavy metals in food have been discussed in terms of inorganic salts in the field of hygienic chemistry, partly because of analytical methods used. However, in order to evaluate accuratly their biological characteristics, such as absorption, excretion, accumulation and toxicity, the chemical forms present in natural food should be identified. In Japan, marine algae are eaten as food. Since they contain relatively high levels of arsenic compared with other heavy metals, the chemical form of arsenic was investigated. Hijiki (Hijikia fusiformis) contains a large amount of arsenic compounds, corresponding to about 100-180ppm as A_S2O₃ in dry samples. Twenty-five percent of the arsenic in Hijiki was readily soluble in water; the remainder was fat-soluble or conjugated with tissue components. The water-soluble arsenic was also soluble in methanol and ethanol, and may consist of low molecular weight organic compounds, based on the results of gel permeation chromatography and ultrafiltration. Seventy-five percent of this material was adsorbed on weak cationic exchange resin (Dowex CCR-2); it could be eluted with 0.01N hydrochloric acid and yielded white tetrahedral crystals, having weak UV absorption at 279nm. High performance liquid chromatographic analysis of this material showed that the pattern of absorbance at 279nm in the elute essentially coincided with that of arsenic concentration. This material, which is a major component of the arsenic-containing compounds in Hijiki, thus appears to be an organo-arsenic compound.